La Ventanilla is a nearby lagoon and mangrove area usually visited by small boat. The appeal is wildlife rather than adrenaline: crocodiles, birds, mangrove channels, and the sense that the coast is more than a line of beaches.
It fits well with Mazunte's conservation story, especially if you want a half-day that is not just sand and sunset. Ask locally about current conditions and community-run tours before heading out.