I have been to La Fortuna (the city where Arenal volcano is) two times. Each time, I stayed at a small, local cabina-type place for a mere $10 a night. The owner is very nice, rooms clean and cheery (no frills), and just a few steps off the main road in La Fortuna. I can’t recall the name of the place, but many such as it exist and you don’t need to reserve in advance- just get off the bus stop and there will be people ready to walk you to their cabinas. Also, there is a restaurant called La Parada (near the bus stop or “la parada”) that has very good smoothie drinks and also meals for next to nothing. Good, cheap eats.

For Arenal, I do recommend the hiking tour to get closer and see a lot of the volcanic rock from bigger eruptions.

There are two major hot springs from Arenal. One is onwned/run by the nicest hotel in the area (Tabacon) and the other is Baldi Termae. Baldi is less expensive and very nice.