Reserva Natural La Cordillera, comprised of Icacos, Diablo, Palomino, and Palominitos, is a protected string of small sandy islands just north and east of Fajardo  with lots of great snorkeling and diving spots around them. Bring plenty of water and sunscreen—there are no facilities or stores on the islands.
To get to Reserva Natural La Cordillera, go to the dock in Las Croabas and arrange a ride with one of the boat operators there. They’ll drop you off and return later to pick you up. The cost is typically $10 each way. The islands can get crowded on weekends and holidays.