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.