Ojo de Agua is the island's natural swimming pool — a spring-fed pond of cool, clear water set among trees on the isthmus between the two volcanoes. The water is genuinely lovely: cold enough to wake you up, clean enough to drink, and deep enough to swim properly. There are rope swings, a small zipline across the pool, hammocks strung between palms, and a simple kitchen for fish and beer.
It gets busy in the middle of the day with day-trippers from Moyogalpa, but go in the early morning or late afternoon and you may have the place largely to yourself. The entrance fee is small and includes use of the facilities. Worth a couple of hours, especially after a long volcano climb when your legs are screaming for cold water.