We're going to Boston next weekend. We'll be there for 3.5 days. We have a ton of stuff already planned for Friday, Sunday, and Monday, but I'm debating leaving the downtown area on Saturday. I'm stuck on Cambridge v. Salem. I want to visit Salem (House of Seven Gables, witch stuff), and my husband wants to visit Cambridge and walk around Harvard, etc. Is it possible or completely crazy to do both in one day? Leave early for Salem, do an early half day there, then back to Cambridge for the afternoon/evening? We generally don't mind long days if its worthwhile. Or, do you have strong feelings of one over the other.
I think you could do both in a day. Harvard doesn't take very long to look around if that's all you really want to do in Cambridge. You can go on a free tour led by a student or basically just wander around the Yard yourself and check out Harvard Square. It's a pretty compact area, won't take more than two hours unless you want to do a lot of shopping in the Square.
Agree with pp about Cambridge. It can be done in very little time. I would consider rolling it into one of the other days and leaving a full day for Salem, however, just because you never know about traffic, getting tired, etc. You can take the T to Cambridge, so there's really very little travel time involved.