I recommend a North End restaurant, especially if you like Italian. North End is within walking distance of TD Garden. Yelp and TripAdvisor have plenty of excellent restaurants.
Faneuil Hall has some unique shopping, especially touristy merchandise.
The weather tends to be summer temperatures still, but if it's a rainy day, Duck Boats or Old Town Trolley will give a nice tour of Boston. Boston has excellent museums like Aquarium, Science, and Fine Arts.
PRomano has some nice Revolution War site recommendations. Plymouth is also an excellent area if you're a history buff since Plymouth Plantation and Plymouth Rock are located there.
Cape Cod is my favorite location in New England, but Bar Harbor reminds me of Cape Cod if you don't have time.