Agree with PHeymont. Plan your trips around the leaf watching. Lots of pretty small towns around New England, although many will be pretty booked.
I'd leave at least 3-4 days for Boston. Lots to see and do there, including many items relating to the rebellious Yankees throwing off the ties with old King George. And great museums and restaurants and such. We can get into more details if you'd like, but you'll definitely want a good guide book.
Many people find a day in Salem (famous for its witch trials) entertaining. I'd add a few days in Cape Cod if time allows.