Be sure to visit the USS Constitution. Lexington and Concord are nearby and are worth the trip for historic purposes. Drop by a couple of taverns in the area to get a sense of what it must have been like at the time of revolution. Cambridge is a busy, jammed up place, but one must visit the Harvard campus. It occurs to me that the Head of the Charles regatta might be around that time. It's something to see.
Salem is interesting, but don't spend a lot of time there. The memorial to those men and women who died during the Salem witch trials is a small spot, but worth seeing. There is an art museum in Salem that is quite well known.
The best North End restaurants are the little ones right on Hanover street, if you can get in. They are jammed with people, we particularly like Carmelitas (sp?), it's small with fast service and great food.
It's worth a walk around Boston Commons. Be sure to find the sculptures by August St. Gaudens. They are near the capital building. St. Gaudens designed America's coinage just after the turn of the last century.
If I think of more stuff, I'll let you know.