The thermal hot springs in Rincón de La Vieja at Rio Negro (Black River) may not be as well known as those in the Arenal area, but we had the pools and surrounding forest to ourselves.
From where we stayed at the Hacienda Guachipelin Rincón de La Vieja, the springs were accessible by horseback or vehicle, followed by a short hike and crossing a rather dubious hanging bridge.
The pools are heated by the Rincón de La Vieja Volcano, so you definitely want to test the steaming water. There are seven pools to sample of different shapes, sizes and temperatures. The first pool was too hot to plunge more than our toes into. And the pool that was "just right" was the last one, which happened to be ideally situated next to the churning river.
After two days of horseback riding, hiking, canyon ziplining and rappelling, it was great to unwind at this private oasis.
If a rainforest/volcano adventure beckons you, I recommend staying at Hacienda Guachipelin Rincón de La Vieja and getting an adventure pass from Adventure Tours--both are at Rincón de La Vieja National Park.
Rooms at the Hacienda are about $79-$96. The adventure pass combines four activities and is about $85 per day.
Stay AND Do: http://www.guachipelin.com/en/tours.html