Heritage Plantation (History - and a great antique car museum ) on Cape Cod in Sandwich -- Nickerson State Park (Great Park, no hookups) in Brewster (Cape Cod - swimming, fishing, biking, boating) ... Plimoth Plantation in Plymouth Mass (History 1600s)... Old Sturbridge Village (History 1800's) --
Sweetwater Campground in Brewster has HU it's central to the Cape sights, but a lot of small sites - so check size.
Falmouth (Cape Cod -- Fishing village, Oceanographic Institute -- if you have kids, do the OceanQuest voyage.
Salem Village (Witches) Parker River Wildlife Sanctuary (Wildlife - North of Boston.
September is early for the leaf-colors ... I agree = mid-October best.