It depends on what you like to do. You'll be in town during the 4th of July fireworks celebrations. Seeing them on the Esplanade can be a bit of a chore since its often crowded, however. If you've got a hotel with a view they're worth checking out.
If you like history you can see the historic sites. You can walk the Freedom Trail, but a lot of it isn't that interesting (and I'm a history buff). I think the two most interesting things are Paul Revere's house and the USS Constitution. If you're willing to go out of the city a little bit then I'd highly recommend going to Old Sturbridge Village. It's a living history museum in Sturbridge, about an hour west of the city. They're a great museum which recreates a New England village of the 1830s. You can also go to Plymoth Plantation, south of the city which recreates the 17th century Plymoth settlement.
In the city I'd recommend going to the Museum of Fine Arts. It's a world class art museum located right in Boston, near Northeastern University. They've got a lot of great art, including some fantastic 18th and 19th century paintings in their American wing, such as Gilbert Stuart's famous portrait of George Washington and John Singer Sargeant's innovative portrait "The Daughters of Edward Darley Boit".