Beantown tourism

So my plan while in Boston has generally been "be a tourist while normal people work, then hang out with folks in the evening." This worked well yesterday (the Arboretum is very pretty; and then I learned to play mah jongg!) and I figured it would work pretty well today, too, since usually when I come to Boston I am not primarily doing touristy things, so clearly there should be plenty of things to see. But, er, looking at the "things to do in Boston" page, it turns out I have seen rather more of them than I realized! At various points in the last ten years, I have visited the ISG art museum, the science museum, the public garden and Boston Common, the markets at Faneuil Hall, and the shops on Newbury Street. It'd be nice to walk the Freedom Trail, but it looks sort of wet today. What else should I see? (If anyone wants to come along, bonus!)