Okay, here I'm copying & pasting the old post. I think the credit goes to CTDC, but I didn't write that part down. Sorry if the formatting is funky.
look at this website and buy products from them:
this is where we buy many paint, paper, etc. supplies and they sell in large and small quantities. I'm not sure if it's less expensive than buying paint, paper, etc. from staples, michaels, toysrus, target - I think it is, though. buy WASHABLE non toxic tempera paint - it's much easier to wash off. also do NOT buy the magenta or aqua blue colors - for some reason even though they are washable they leave a residue of color on white walls, hands and clothing. the other colors don't, but aqua blue and magenta do. wierd.
lots of great ideas added above, I'd add doing lots of open ended painting and gluing projects - rather than making a 'something' just buy paper in long rolls, cut to fit kitchen table, give them each a brush and a small amount of paint in their own paint cups and let them go to town. if one has red the other uses blue when they mix together they'll get purple... science! blue and yellow make...
when you do open ended activities you aren't focusing on the right way, or even having a right way - they are simply learning and exploring the various media. markers, crayons, tempera paint, water color paints - all do different things and produce different effects - it takes lots of experience and kids love just using them.
do buy a few chubby paint brushes, but you can also paint with other things, like sponges (small), plastic forks, pinecones, small plastic cars and trucks. if you use black paper with bright colored paint and use forks for painting you have art worthy of framing (great father's day gifts)
save catalogs and calendars for kids to rip to shreds now - and start using small (blunt end) scissors for cutting. yes, cutting... the rule is they must sit still, and you sit with them - your daughter might enjoy this, your son would probably get into the ripping.
have a shredding party with newspapers - toddlers just love to rip stuff, and this is legal ripping.
buy a cheap shredder and after you shred various documents - or just put colored paper through the shredder - you get really fun stuff to glue onto paper!
also, take plastic cars, legos, bristle blocks, etc and for painting - cars make great tracks in the paint, and legos etc. make great prints. it's interesting for children to see their legos, cars, in a different light. cookie cutters make good prints once children get used to doing it.
foot painting - but only OUTSIDE with twins! put paper down in the driveway, a low baking dish with just enough paint to wet the bottom, then they step in and walk away. you can do this in the kitchen, and we do this with about 3 or 4 children at a time, but it's messy and our center is designed to get messy - your livingroom carpet is NOT. and with twins.... I'd not try this inside yet.
buy clear contact paper and they can stick lots of things to it, including collecting leaves, sticks, pine needles and stick them on their collage. using contact paper makes it less messy. the shredded paper is fun to stick on contact paper. they can rip tissue paper and stick it on the contact paper, then hang those in the windows for a stained glass effect. you can buy foam shapes and they can stick them to contact paper... and when you're ready, use glue. children have a hard time with glue, because it doesn't stick instantly, and they get confused. use paint brushes to spread the glue on the paper (it's too hard to squeeze those bottles at this age) and then they can stick anything to the paper.
you can buy dot paints which are fun to use - they are like bingo paint dots used in bingo halls, you can either just buy the bingo dot painters or refillable ones (discount school supply sells these, we buy them). I sometimes find the refillable ones aren't all that great, just easier to buy various colors of bingo dot paints. but they are cool and you can HIT them to the paper and make splotches! but you can also drag them on the paper and make lines... very satisfying.
put down contact paper upside down on the kitchen floor (you'll need to tape it to the floor with masking tape or clear packing tape) and walk over it in bare feet, then also drive trucks over it - stick legos, etc. to it - this is a fun and less messy activity.
texture activities are also fun. buy them each a low dishpan (so they don't have to share and have enough space to work) and fill with flour, oatmeal, uncooked noodles, rice, beans, lentils - it's so much fun to feel a texture, then use small scoops and spoons to fill and dump, and small trucks to drive through the texture. they just feel really good and children love textures. plus, different things behave differently with spoons, scoops, cups, etc. like using a sandbox at the park, only inside. you can reuse the textures - just store in a container that is labeled - for play only, not for dinner (ugh). for children who are frustrated, angry, hyper - texture play will usually calm them down. it works for mothers and teachers - plunge your hands into a bucket of beans and your blood pressure instantly decreases, it's amazing! that's also true for playdough, but you said you do that already.
if you mix cornstarch and warm water you get ooblick. go slow with the water, you don't need much - it will look like white liquid on the top, solid under the white water, and when you take it in your hands and roll it or move it, you can get it to form a ball, but when you stop moving it it turns back into liquid. SO COOL! each child gets their own in a small bowl. when it splashes on a shirt, floor, table, etc. once it dries it just brushes off so it is really easy to clean up. just let the lumps and mess dry in place, then sweep away. don't add food coloring to this, though, because the food coloring will ruin clothing and stain tables, walls, etc.
cook with children - if you can stand it, which i cannot, but cooking is science at it's best. and it's science - adding vinegar makes things foam up, liquid when added to flour changes the texture, cracking eggs is cool - you can make cookies, dump it cakes, banana bread and other easy things. they can do lots of the work, like stirring, dumping after you measure it, pouring the liquid in, etc. all of this is good for cause and effect, fine and gross motor, delayed gratification, etc.
buy bean bags and music for listening - can you put the beanbag on your arm? on your shoulder - on your head - you can lead it at the beginning, then there are CDs with songs do this, too for older children.
play simon says - only you won't try to "catch them' yet - simon says touch your head, touch your nose, wiggle your fingers...
I also made up a chant and do this whenever they have to wait - tap tap tap your nose, tap your nose, tap your nose, tap tap tap your nose, tap tap tap. wiggle, wiggle, wiggle your fingers, wiggle your fingers..... ad nauseum. it keeps their interest, and they are learning various body parts and actions, too. then they can take turns picking something to do and being the leader.
buy rhythmn sticks and the CD that goes with them for tapping and listening skills. these are NOT to be left down all the time, but when you are guiding their play.
buy inexpensive and not heavy flashlights, turn off the lights and have light shows. kids love to shine them and see what happens. you could introduce shadow play, too, if you have a wall they can see their shadows on as they wave their arms, touch each other's arms, their head, etc.
paint on shoe boxes, rocks you collect outside, toilet paper and paper towel rolls, small boxes, etc. if you have LARGE boxes it's really fun to paint on those - since you get to stand up to paint.
you may want to buy them an easel, each child paints on their side for days when they juts can't get along. at this point put out 1 or 2 colors each, they can increase to having more colors as they get used to painting and staying STILL at the easel.
biuy small chalk boards and their own erasers and chalk (both sidewalk chalk and regular sized chalk) to use at the table.
markers and crayons - your son might like washable markers more than crayons as markers require less pressure to make your mark. even just taking off the covers and putting them back on develops good fine motor skills.
buy small rolling pins, plastic cookie cutters, plastic knives (they are very safe) and plastic forks for playdough so kids can practice cutting, printing, rolling, squeezing, pinching, pushing, and learning about part-whole relationships.
yes, buy duplos, bristle blocks, wood blocks for open ended building - buy match box cars so you always have enough cars and can make a garage. buy small animals so you can build a zoo.
they can make instruments out of rice, lentils, beans and put into plastic water or juice bottles, seal with electric tape or glue the covers back on if you feel like they'll open them later - we have these out all the time for exploration in both infant and toddler rooms (we do seal the lids so it's not a choking hazard) - different amounts and different things make different sounds. Also, make swirl bottles - a bit of food coloring in water, then add a little bit of cooking oil or mineral oil to the water and when you shake them they are different than shaking colored water. you can add plastic items inside, too, that will move around when you shake them... or add some glitter.
stand at the big kitchen sink on chairs and fill it with water, small boats, funnels, scoops - water play is a HUGE hit. take their shirts off and let them go to town! sometimes you can put some bubbles in, sometimes they can wash their dolls or a few toys, other times just put various sized funnels in plain (warm) and they'll love it.
buy small spray bottles and fill with very watered down paint - and spray on paper, or spray on paper in the middle of the grass outside. it's fun to make splatter paint! add liquid dish soap to the water and it's easier to clean if they spray hits the floor.
oh, save some diaper wipe boxes, wrap paper inside, put a golf ball inside with a blob of paint and SHAKE! not only do you make a cool sound, use your large motor skills to SHAKE, but when you open it the ball has made interesting patterns on the paper.... you can use shoe boxes the same way, and they can take the cover off and ROLL the box to watch the ball making tracks. if you get a larger box they can work together, one on each end, to lift the box up and down, tilting it to watch the golf or tennis ball make paint tracks. too fun.
another hint to extend toddler's interest in an activity: don't give them all the stuff at once. for example, get out the playdough and forks. as they start to lose interest, but not before they do, add cookie cutters, or plastic knives, or legos to make prints. if you toss it all on the table they tend to use each thing once and then want to move on. if they have 1 or 2 items they investigate those, you can encourage them to try other things so they develop other skills or investigate ways of using a tool they haven't yet. Use leading questions rather than showing them how to do it - sometimes you want to instruct, but mostly you want to encourage them to work something out, experiment, etc. Questions like Can you cut like this? i wonder what happens if you push the legos down HARD in the playdough? oh, DD, can you do it like DS is doing? I'll make some balls for you and you can squish them.... with a paintbrush, you can ask them wow, can you make looooonng lines? thin lines? thick lines? .... not always asking these type of questions, mostly you just want them to explore themselves, but after they have lots of experience and they seem a bit bored with plain ol' brushes, you can challenge them... can you make really thin lines? thick lines?
oh, you can also paint with toothbrushes, or combs, or apples cut in half. the trick with painting is not to put too much in the cup or bowl, have them sit across from each other at the table, and have you sit with them, too.
yes, invest in smocks, or just take their shirts off.
oh! use cool whip or shaving cream for fun texture play! jello is a fun texture activity, too - they can make some to eat for dessert tonight tomorrow, but another color they can play with. and if they eat it, who cares? if they still put lots of things in their mouths or are apt to get it on their faces (smearing faces is fun!) then you should use cool whip rather than shaving cream.... the whipped cream from the spray can is the most fun because of the fun way it comes out of the can. also, getting that edge gel that changes from green gel to shaving cream when you touch it with wet hands is FUN! again, only if they won't rub it in their eyes or eat it...
oh, you can also paint on tinfoil, wax paper, fingerpaint paper, sand paper for another texture - the paint works differently on different surfaces.
oh, and outside, you can paint with plain old water on the house, wall, sidewalk, anywhere - it's just water so it dries but it's cool!