Where to Stay
Stay at Arenal Backpackers Resort. It has a pool, a bar, hammocks, and is generally conducive to meeting other travellers. You feel like you are staying at a real resort, but only paying around $15 per night. They also offer camping in tents!
Where to Party
The low rating comes from the fact that there’s pretty much no good bars in the city. You pretty much have to round up a crew at the hostel and choose a dive bar to go to. There’s a big bar/club there called Rockoco, but is usually pretty dead. The only reason this city has any rating is because the hostel has a great bar and if you are lucky and put in some effort, could be a pretty rowdy place (and the staff will fully support it).
During the Day
People come to La Fortuna for the daytime activities. These include hikes to the volcano, white-water rafting, canyoning (AWESOME), etc. You can book these through the hostels or one of the many vendors on the street. This city is somewhat similar to Banos, Ecuador.