I would push doing the "Pilgrim thing" if you are going to be in the area of Boston / Cape Cod. The boat Mayflower II, Plimouth Plantation, etc. Note about Plymouth Rock: It, literally, is just a rock (lol!). Many people just look at it and walk away. ASK one of the Park Rangers there about it - a very nice Ranger there gave us a fascinating 10+ minute talk about the history of the Rock... even our young kids were impressed.
If you are in Boston, Boston Aquarium is very cool. They have a huge center tank that you walk around on a ramp. Part of one day is enough time to plan for it.
We have not done it, but have heard lots of good things about the "Duck Boat" tours.
Cape Cod - we camped at Atlantic Oaks, a very nice campground in the middle of the Cape and directly on the "Rail Trail". If you can, bring your bicycles! The Rail Trail is an old railroad bed paved over for use (mostly) by bicycles. Long, straight and flat runs with few curves or very gentle hills.
In Maine, Acadia is really awesome... for adults and older kids. We've been there a few times and are planning on going back. It is all about hiking and bicycling. You certainly can drive to most of the major spots there, but I am just not sure how much "fun" a younger kid would have there.
Upstate New York - My son and I greatly enjoyed Fort Ticonderoga at the northern end of Lake George in New York - we are history buffs. We stayed at Lake George Escape Evergreen Campground, which was big and had tons to do in the campground itself. It has been years since I've been to that campground, but I've read it is still good. It is at the northern end of the lake, and son and I found the scenic drive up the lake on rte 9A (I think? NOT the highway!) to be surprising fun!
Niagara Falls was neat to visit, but we didn't find it "awe inspiring". There is a LOT to do in the area for kids though, as it is a huge tourist area, especially on the Canadian side. After saying the falls are not awe inspiring, I do however recommend you definitely do the Maid of the Mist boat. We enjoyed dinner at the Skylon Restaurant overlooking the falls. If you take your camper across the border into Canada (we found the KOA on Lundy's Lane to be great), look up info on crossing the border - lots of tips and things to do to make it easy and smooth.
Gettysburg Battlefield in Pennsylvania was impressive, and worked out well with kids because you can do the "self drive tour" using CDs in your own vehicle. We made it more interesting for the kids by finding the monuments to men from our state and other states that we had visited.
In Connecticut you have Mystic Seaport and Gillete Castle (if you have time for it - the castle is a one day visit, but may be out of your way). Mystic Aquarium is okay, I am not too impressed with it, but I haven't been there in years. Connecticut is neat for adults to explore, but not a whole lot to keep a young kid's interest.
Have fun!