Depends of couse on what you like to do.
We toured RI last year and we spent a few days at Burlingame state park followed by a few days at Charlestown breachway. The beach being a short walk from our site at the breachway was nice. The stone / rocks surprised us at the surf edge but once past that no problem.
If you stay at the breachway (dry camping) you may use the facilites at Burlingame.
If you paddle (we canoe) Ninigret pond is nice. We saw oyster farming for the first time up close. Horseshoe crabs moving under us. We put in at Ninigret park.
The park is a hidden jem, check it out on Friday night, the free observatory was worth a stop.
They also have a neat little swimming hole with lifeguards.
Slater mill http://www.slatermill.org/ is a neat few hours.
The hot air balloon festival was neat http://www.southcountyballoonfest.com/
As other mentioned the cliff walk is a nice peek at what you can't afford. Or maybe you can but are smart enough to have a camper instead.
