Wherever we go, NY, SF, Rome, Paris, Barcelona etc., I recommend a Hop On and Off Bus. Go on the whole trip as a reconnaissance & then again picking spots where you wish to get off & then on again---much cheaper than taking taxis.
I write this having lived in the SF Bay area for 40+ years but 1st 21 are in Maine & military called me away. Still come to New England once per year as have family in Portland, Portsmouth & Newburyport(someone mentioned that town & great seafood restaurants & bay views).
Someone also mentioned the Freedom Trail---Look it up on Google On it you'll see Boston Common, Ben Franklin statue, site of Boston Massacre, Paul Revere's House, The Old North Church, the USS Constitution, the Bunker Hill Monument etc. and more. Boston is a great walking city and one neighborhood you might wish to see is Beacon Hill with narrow, tree shaded streets lined with old brownstones and townhouses. Always, one of our favorites is the Union Oysterhouse, Boston’s oldest restaurant, touristy but still VG. I still have a T-Shirt, " Eat Oysters, Love Longer". http://www.unionoysterhouse.com/ .
If you need some Carbs after the walk, the NorthEnd Italian section on Hanover St has a plethora of pasta/pizza & garlicky places !!!
If you make a trip to SF, a great walking town too and quaint like Boston as we are the West Coast Bay Area, I will give you an itinerary.
SF Giant(AT& T) park downtown like Fenway is awesome !!!