One of the best experiences I ever had on a vacation was in Monteverde. If you're up for a little adventure and not afraid of heights, consider doing the Monteverde Canopy Tour.

After booking your tour, the company van will come to your hotel at the designated time to pick you and other tour members up at each hotel. The van then drives to the company's own private reserve, located 7 km outside of town. You'll be fitted for a saftey harness and the guides will explain how to control your speed, etc.

Soon you'll be clipped onto a wire, zipping from platform to platform like the monkeys who are probably watching from their own perch. There are 14 platforms total, which span over a mile.  

The phone number is 506 645-5929. Allow about 2 1/s hours, including pick up and drop off.

If whisking your way through the trees sounds a little too adventerous, there's also the Sky Trek. This is also high up in the canopy, but it consists of a series of suspension bridges about 130 feet in the air.  This is a more leisurely pace.

If neither of these sound like options for you since you have no plans on being over 100 feet off the ground unless you're in an airplane, you may want to visit the nearby Butterfly Garden which displays both mounted and live specimens.

One remark: if you want to see the Volcano Arenal, Monteverde is a little far (the roads are bad and a trip that would take 2h, it may extend to 6-8h)... the volcano is a great experience and there is a lot of cannpoy and other eco-adventures close to it.