Mystic Seaport for sure! Another decent option for that area is the Mystic/Stonington KOA in CT (should you not wish to camp at a casino).
There is so much to see & do all along the east coast - so much history especially. Another do-not-miss IMO is Colonial Williamsburg, and yet another is Gettysburg. I'd recommend at least a full day for each one (more if you can arrange it, especially in the Williamsburg area, as Jamestown & Yorktown are also in the same local area. CGs have changed since we were in VA, but for Gettysburg, we enjoyed the KOA there.