I also visited Costa Rica on Rustic Pathways dime and due to all the volcanic and tourist activity, hot springs are super popular.  I may have gone to a resort as well one night, but I am having trouble remembering which one it was.  This one is free and totally natural, just a river that is hot in some spots.  I had a blast here playing with plants and finding lizards (attaching some pics here).  There were some people around but you can hike up the river quite a bit to find some solitude… I did not go very far.

—Spa Thought Partner Stacy