score:7
A good starting point is the Freedom Trail -- it's a nice primer on Boston history. They also offer walking tours. The trail includes:
Beyond that, depending on your interests there are numerous other options. You could visit the Harvard University campus in Cambridge (an easy ride on the T red line), Fenway Park, Newbury Street (some typical Boston architecture, plus a decent choice of shopping & restaurants), etc.
The Celebrate Boston site is a good resource for what's happening in Boston and what's worth seeing. I know that you mentioned historic, but worth mentioning Arts Boston for additional arts-related listings.
Also, have a look at MassHome's Boston Page. Not very pretty, but has lots of links to different tourist sites in Boston.