Is a must do. A 20 minute walk NW from Seven Seas public beach. You will hear the Atlantic Ocean while you are protected by giant rocks inside a natural salt water jacussi! There a few of them. The last one is the largest. Go to Playa Colorada (Red Beach), you will see the rocks at the end. Hike to the top of the hill and you will see El Yunque, National Rain Forest ( a 20 minute car ride from Fajardo). Also will see the light house and Las Cabezas de San Juan Reserve.
Go to the Seven Seas beach. Wear good sandels or shoes. Hike around the point to the left, clambering over the boulders. About 1/2 km around the bend you can find some nice secluded beaches and many of the younger, more 'liberal' minded folk tend to congregate here, far from the eyes of parents. Just be cool with the scene.