Another vote for Freedom Trail in Boston. I've been to Ben and Jerry's twice, 18 years apart. If you are going to Stowe anyway, it's worth the stop. I wouldn't drive out of my way.
It's a pretty good haul from Niagara to Boston. We left Niagara Falls and made it to somewhere in Eastern Mass. I remembering researching nice stops in Eastern NY but we didn't have time to stop so I don't remember details. We needed to go to Newport, RI to drop gear off and then headed up to Vermont. We've stayed at the KOA in Queechie Pines twice. It's centrally located for day trips in VT and NH. If you are interested, I can look up the map and tell you which site to book.
Here is a state forest 20 miles north of Boston. It's beautiful country with multiple lakes and lots of hikes. I haven't stayed in the campground but I'll post the link. It may be an option for camping outside the city or it may be a destination on its own. Definetly want to do park and ride into the city. You can walk the Freedom Trail in a day including touring the USS Constitution.