This place is an absolute find - deep in a jungle, in the middle nowhere. Twenty minutes outside of San Carlos, and near the base of the Platanar Volcano and on the edge of the Juan Castro Blanco National Park, are the Termales del Bosque, or "forest springs".
Hop in the pools of progressively hot waters as you sip on an Imperial Beer and listen to the Blue-Crowned Motmot sing. Try to go when it's raining. The tree canopy protects you as you listen to the symphony the heavy drops and massive leaves create.