Arenal is too far from Monteverde to do as a day trip, unfortunately. If you have 3 nights, better to stick with Monteverde if you’re set on that. The drive time between destinations is about 3-4 hours because the road is very rough for a portion of it. There’s also the Jeep-Boat-Jeep across the lake but that still takes a few hours. Not sure how set you are on Monteverde, but the Arenal area has a couple of different options for hanging bridges too if you really wanted to visit the hot springs (which are awesome). Of course then you’d miss the cloud forest…so I guess it really depends what your priorities are 🙂
Answer for Monteverde or Arenal Volcano???