Chicos is great, and the view of the river is worth it alone. Food was excellent, and you can watch the kids dive into the waterfall for a few pesos. (About 40 minutes away.)
Playa de Mismaloya is a really nice beach and used to be a quiet, fairly remote cove. It is still fairly remote, and is worth the visit if you want to go to the beach after Chicos. Try to sneak on to one of the Hotel properties so you can use the pool or facilities. Just act like you belong, it works. Have a good time.