We just got back from 4 great days at Nickerson State Park on Cape Cod I've bicycled by it many times but never camped. What an amazing place, 2000 acres, 400 campsites all hidden in a forest. Miles of bicycle/walking trails within the park plus the Cape Cod Rail Trail (20 miles long) right at the entrance and several kettle ponds for great swimming kayaking whatever. Clean bathrooms and free hot showers.
The bad no hookups, next to impossible to get reservation in season but if you can go before Memorial Day or after Labor Day it's a campers dream.
PS: Before you ask I had to look it up too:
Kettle Ponds:
A kettle (kettle hole, pothole) is a shallow, sediment-filled body of water formed by retreating glaciers or draining floodwaters.