Gilligan’s Island and nearby Isla Ballena (Whale Island) are all part of the same set of small mangrove islands off the tip of the Caña Gorda peninsula and are technically part of the dry-forest reserve of Bosque Estatal de Guánica. Neither is too sandy, but both offer good sunbathing and passable snorkeling, and both get quite popular with day-trippers.
The ambitious can reach both islands via kayak; rentals are available at the Copamarina Beach Resort and Mary Lee's by the Sea for $15 per hour or about $35 per day, or you can catch the ferry in front of Restaurante San Jacinto every hour, barring bad weather.
If Gilligan’s is packed, pony up a couple more dollars to the captain and try the less-visited isles of Ballena. The isles are closed to the public on Monday – apparently to give them a chance to recover from the influx of tourists over the previous week.