Sendero los monos
Hike to Hidden CoveNorth of oft-visited Manuel Antonio on the Pacific side of Costa Rica, is a small cove with a public beach by the name of Biesanz. The beach is hidden from the cliffs high above where resorts are like stars in the sky, numerous and crowding. Hike down a path that's largely unmarked locally, yet discoverable via Google or Mapquest, and you'll be rewarded with a beach that rarely hosts more than a dozen people at a time.
It's just popular enough to include amenities like kayak and canoe rental, even snorkel gear. Just get there early and plan to only stay for the hours the sun is directly overhead. Once the afternoon wanes, the cove becomes rather dark because of the high cliffs overhead which act like a natural shelter.
Added bonus: the sand turns purple here in patterns that recall Marimekko or Ikat designs.