Go here. Stop researching and please just go here. This place is unreal - beautiful, fun, romantic, and relaxing. There are so many different hot springs and waterfalls to explore and you can also have lunch/dinner. We had lunch and it was very yummy. Earlier in the day we went horse back riding near the arenal volcano and it felt SO GOOD to dip in the warm natural pools after all that activity. There is also an adult only 'garden' with lounge furniture where you can continue exploring the hot springs and also grab a drink or a nap ;)
Tip: go during the day but stay after sunset. The place is so fun to explore at night as well!
This was the absolute highlight of our costa rican vacay!