Very close to Boston, in addition to Walden Pond I would recommend taking the part of what I think they call "The Freedom Road" and visiting Lexington and Concord. The town green in Lexington (I think) where the first battle of the revolutionary war occured. It is now a nice example of a small New England town. And then the bridge where the second battle occured outside of Concord and where the poem about the battle was inspired. It is a beautiful country are apart from the historical importance.