Depends what you’re into and where you’re coming from.
if you’re into history, there’s the site of the Boston Massacre, with the Old State House adjacent; also Faneuil Hall, and the Boston Commons. (Faneuil Hall had a small museum upstairs and an open-air mall adjacent with lots of food places btw) if you like cemeteries, there are colonial-era examples by the Commons and in the North End, there are several historical churches downtown as well. etc.
if you’re into colleges, the Harvard and MIT campuses have some nice buildings, you can usually wander around there during the day.
see if the theater district is showing anything while you’re there. it’s not Broadway, but it’s not bad either.
the Boston area has a wide variety of restaurants from different countries.
think about your hobbies or crafts. then search for a store or club or organization related to that in Boston, there’s probably something related. for example there were several small places that built/repaired antique musical instruments, several gaming shops, several shops for stamps and collectibles etc.
Finally, Boston has a variety of “tour buses” with various themes that drive around downtown, and some that go in the water, they can be worth checking out.