Wompatuck State Park is between Boston and Plymouth - easy drive to both. There is also a ferry in Hingham that will take you to Boston - cheaper and easier than trying to park. We were there last week. Nice enough campground with very separate sites. 20A electric with water scattered around the loops. We would have enjoyed it more had it not been for the caterpillar invasion. If you sat in one spot too long they decided you were a tree. We were there last week; the caterpillars should be gone soon.
Scusset Beach State Reservation is very nice. We stayed two weeks and enjoyed it a lot. E/W hook ups. Only bad thing - the dump station has limited hours (closes at 3:30PM) and so you must plan to hit it during open hours. The campground is right on the cape cod canal and bay. Easy drive to Plymouth and Cape Cod towns.
I would recommend Fishermen's Memorial State Park in Rhode Island. They have full hook up site in a lovely location not far from the beach. Narragansett is a pretty town. Not too far a drive is Mystic, CT; Providence, RI; and our favorite, Newport, RI.
Right now, we are in Salisbury Beach State Reservation. E/W, open sites next to the beach and river. We are driving distance from Boston, Salem, Gloucester, and Concord, MA (lots of history) and Kennebunkport, ME and Portsmouth, NH. Lots of history in the area...