Looks really fun good. A few suggestions: skip Plymouth and go straight to Newport, spend the one full day in Newport touring one or more mansions, maybe go over to Jamestown. I would only go to Providence for dinner.
Burlington seems too far to go; you can go to Hanover, NH and see Dartmouth College and walk around. Stowe/Ben & Jerry's/Warren is a good idea.
Freeport is just north of Portland and good for shopping in any weather.
I like Portsmouth a lot.