Playa Platanares is the easiest beach escape from Puerto Jiménez, reached by road or a long hot walk from town. It faces the gulf, so the water is usually calmer than the Pacific side of the Osa Peninsula.
The beach is better for wandering, birdwatching, and a quiet swim than for big surf. Bring water, watch the tides, and do not expect much in the way of services once you leave town.