I work in Boston and live about 12 miles away. Clori has given you excellent suggestions. The Swan Boats in the Public Garden are inexpensive and unique. The Freedom Trail is great for history buffs. When I first moved to Boston 32 years ago I took the Old Town Trolley tour which is a great way to start your trip. We also have several world-class theaters if you want to take in a show. We have so many restaurants to choose from that I don't know where to begin - I would start with TripAdvisor - its pretty accurate and I have posted reviews there.Are kids going and if so what ages. I live near Boston so don't eat there that often but you can go to Faneuil Hall which is basically a giant food court. You can also go to the North End and get pizza slices for lunch etc. There is a brand new place called Eatily at the Prudential Center that has some casual food (I haven't been yet). The Cambridge Side Galleria also has a food court.
There are many attractions depending on your interests. There is the Museum of Science, Museum of Fine Arts, Institute of Contemporary Art, Children's Museum, Aquarium etc. For history there are places such as old burial grounds, Boston Tea Party Ship and Museum, Old State House, Old South Meeting House, USS Constitution and Museum etc. For Sports there is Fenway Park which may have tours and a small Sports Museum. The Boston Commons has a big splash pool and playground. The Boston Gardens has a Swan Boat ride. The Duck Boat is a land/sea tour. Nearby on the subway you can get to LEGOLAND discovery center and Boda Borg.
I second this recommendation of Union Oyster House. I would not, however, recommend Legal Seafood which has locations all over Greater Boston - the food in my experience does not match the price.The Freedom Trail is worth checking out. Also the Union Oyster House. Its on the trail. Oldest operating restaurant in the US. Plus excellent chowder.
We bought the Go Boston card and enjoyed many attractions on the card. Had a beautiful day for whale watching. See if the Red Sox are in town. It was very easy to get to on the T. Also enjoyed the Kennedy Presidential library/museum.
As far as food, definitely go to the North end. Tons of Italian restaurants and try Mike's Pastry or Modern Pastry.
There is an open air seafood place very close to the Children's Museum that is right on the water that we always eat at our first day in town--I can't remember the name of it right now.