Salto de Malacatiupán is just outside the town of Atiquizaya in Ahuachapán department. The river here is fed directly by the Ahuachapán geothermal aquifer — so the water cascades into the swimming pools at roughly 38–40°C (100–104°F) year-round. It's one of very few naturally hot waterfalls in the Americas.
Entry is $1.50 cash for adults 12+; parking $2 cars / $1 motorbikes. From the parking lot it's a ~10–15 minute walk down a steep dirt path to the falls — slippery in rain, sturdy footwear needed. Open daylight hours roughly 7 AM to 5 PM.
Best visited in dry season (November to April) when the water runs clearer. Weekday mornings avoid the local crowds that fill it on weekends. About 1 hour from Barra de Santiago, 1.5 hours from Caluco.