So I just got back from La Fortuna after 3 nights/4 days and loved everything but the hostel I stayed at. I wrote a review detailing my exact experience which was quite negative, and the hostel manager responded with accusations and lies and basically said I'd lied about the entire experience. Not sure what else I can do to refute this...? Just wanted to mention that if anyone is thinking about staying in a hostel in town, please don't stay at this one. The manager is incredibly nasty and I'm still shocked by his response.
Besides this, I really enjoyed my time in Fortuna. Thank you all for the advice on Arenal Observatory Lodge- ended up spending the morning there and hiking all of the trails other than Chaco Chato, which was closed. All pretty short, easy hikes, but still very nice. I took the shuttle (info on the AOL website) and since the afternoon shuttle was incredibly late in the day, was able to share a taxi with some others back to Fortuna. For the Fortuna Waterfall, they open at 7AM and I took a taxi there at 7:30 and was the first one around. Had the waterfalls to myself for a good 30 minutes before another couple showed up, which was awesome. The river was nice for swimming as well, though cold that early in the morning! I actually walked back into town from the falls, which was all downhill and a really nice walk with a stop at the chocolate tour/farm. About 2.5km, I believe.
Also went to two hot spring resorts, Baldi and Ecothermales, both from around 5PM to 8PM. Much preferred Ecothermales as it was a lot more natural, but also more adult friendly with less kids around. I was surprised to see even in the highest hot spring pools had a large number of toddlers and small children. Which is great if you have kids, but not super relaxing if you don't! Just an FYI. For meals, Soda Viquez and Soda La Hormiga were great (especially the latter!) and cheap too. Super Christian 2 was convenient for self-catering. Taxis were $8 each way every where I went and after a day started taking unofficial ones, which were $6.
Anyway, other than experience at the hostel, I had a great time in Fortuna and thank you all for the advice!