Like Monteverde, there are about a bazillion tours available in La Fortuna, mostly visits to Volcan Arenal. We opted to simply take the one suggested by our hostel. It was $40 USD per person, and included transportation to the volcano, a small hike, a dusk volcano viewing, a trip to a local hot springs, dinner and transport back to the hotel. It seemed like a good deal, and it was.
Our tour went to a privately owned observatory, where we and seven other folks walked through the rainy forest to a very pretty waterfall. We then hiked back and hung out at the hotel on the property, watching the cloud-shrouded volcano carefully for signs of lava flows. We did see a few bright sparks that were molten rocks bouncing down the sides, but the sound of the lava flow was much more spectacular than the views. It was like listening to popcorn popping right next to you, and we were several miles away. Very impressive.
We were then taken to Baldi hot springs, which is a hotel and resort with several pools ranging in temperature from hot to way too hot. We had a nice buffet dinner there, then spent a few hours mincing around between the pools trying not to get lost or cooked. It was a great way to wind down after a couple of days of hiking, though there was very little of nature visible at that facility.