Preciosa-Platanares Wildlife Refuge protects coastal habitat just outside Puerto Jiménez, including mangroves, beach, and forest edge. It is a good low-key place to look for birds, reptiles, and the small dramas of the tidal zone.
Use it as a gentle counterweight to Corcovado rather than a substitute. The refuge is close enough for a short outing, but the heat and insects are real, so go early and carry water.