We don't have a camper, but we have kids. When mine were as small as yours, they were really happy swimming. Hotel pool or beach. They loved both. They also really liked little amusement parks and fairs. And farms and zoos. With little kids, think little events or places. Things that are not the really big Disney/Six Flags type of experience. We saved those for when they are bigger.
We live near Lake Michigan and have found some wonderful beaches here. We have gone cherry picking with them while on vacation. We visited a petting farm and went to a county fair. They were really happy with this vacation.
We're very fortunate in the area where we live. We're about 15 miles from inland bays and the Atlantic Ocean to the east and 10 miles to the inland bays and Chesapeake Bay to the west and there is some type of fair or festival here every weekend starting like next weekend and going through until October and nothing really more than an hour away.............we do lots of day trips :)
Lake Michigan has some gorgeous beaches--but it is easier for us to visit the ones on the mitten side!