Easy and obvious Celtics to do would be to go to the Garden (aka North Station on the T, last stop on the green line), as well as check out Russell's statue (which is not at the Garden, it's near City Hall).
Tons of non-Celtics history to see in Boston (obviously). If you're into Ralph Waldo Emerson, you should check out Walden Pond, all the Revolutionary War stuff... etc:
Take a ride on a Duck Boat?