Arenal Backpacker's Hostel beats most five star hotels in Costa Rica. You can purchase a private room, dormitory, or tent for less than 30 dollars. (Don't assume the tents are the worst option simply because they are the cheapest: they include cushy and clean single mattresses). There's a bar, free wi-fi, and a curvy pool with a swim-up bar; the staff is friendly; and the place attracts people from all over the world.
The best part: you will be smack in front of the stunning Volcán Arenal.
Warning: although the hostel is inexpensive, the restaurants on the main road are pricey. Venture to one of the side streets for a tasty and cheap bite.