I'm so happy you are enjoying your babies, and so happy for this chance to plug some books I like. As far as toys and activities, you probably know that they still take the most delight in you -- your facial expressions, your songs, your hand games, anything you do that engages with them -- but as they get more mobile and curious about their environment, they need toys that interact with them. I'm not talking computerized toys; I'm talking about the old-fashioned kind of toys (Jack-in-the-Box comes to mind as I type this; yes, really) where their action gets a reaction. It's how they learn cause and effect.
Books, in no particular order:
1. "What's Going On In There?, How the brain and mind develop in the first five yars of life," by Lise Eliot, PhD. As one reviewer writes, this book is "a popular science book that's a real page turner."
Eliot's point is that the brain's development is based on experience, a notion I find quite compelling.
2. "Your Child's Growing Mind, Brain development and learning from birth to adolescence," by Jane M. Healy, PHd, covers a lot of ground but I have always found Healy to be a great resource. She, too, stresses, hands-on, down-on-the-floor-time with your babies. She's written several other books, all of them excellent.
3. "You Raising Your Child, The owner's manual from first breath to first grade," by Michael L. Roizen, MD, and Mehmet C. Oz, MD, is very accessible and has a section on "Building the Brain" (although it's more of a good over-all book on development) and gets into the nitty gritty of play in a generic kind of way rather than recommending specific toys. Here's one point the authors make about play for 2-year-olds: "You've got to keep the stimulus [ie., toy] novel enough so that your child stays interested, but also familiar enough to give her the time to practice and then master fine motor skills."