Selden Neck State Park is an island in the Connecticut River with no roads or development other than 4 campsites. You have to kayak or boat in. There's a boat launch across from the island on the west side of the river at Deep River Landing where you can leave your car. (Entering and parking at the boat launch requires a permit Memorial Day to Labor Day.)
It's about a 20 minute paddle across the river to the island. If it's a nice day, you're going to encounter some wake from passing boats so make sure your gear is in waterproof bags. Once you get to the island, follow the bank north until you spot the sign for your campsite.
My 5-year-old and I camped at site 2, Hogback. I haven't visited the other sites, but probably will just book this one again because it was so good. Plenty of flat area to pitch a tent, nice picnic table and fire pit. There's a private outhouse. At low tide, a couple of beaches emerge to play on.
The sunset view was amazing. There was a little noise at night from boat traffic but it didn't keep us up. In the morning, we were completely socked in with fog which would have been an issue if we had to get back early. But it burned off before long and we headed back.
