Faneuil Hall. Walk around Harvard Square. Freedom Trail is also good for history loving tourists. I believe they do tours, so you can check one of those out. You can also visit some of the colleges here to see the buildings.

There are also the museums (Museum of Science, Harvard Museum of Natural History, Museum of Fine Arts, Isabella Stewart Gardner Museum).

Are you going to be driving? Because if you go to Salem, it will take you about 40 minutes to drive there, depending on traffic. But you can easily spend a whole day there. But I won't consider that as visiting Boston, if you know what I mean.