The Arenal volcano and the town nearby, LaFortuna are great places. We got an incredible deal so while there we stayed at the Tabacon resort. I don't recommend dropping that kind of cash for a room though, but a day pass to Tabacon's hotsprings would definitely be worth it.

We traveled in a rented SUV and must have stopped at 30 or more Sodas along the way. 30-40 miles of Costa Rican road...Tico Tacos and an Imperial just seem to go together.