I recently made a stop in Monteverde while traveling through Costa Rica with some friends. Monteverde is an incredible location, worth far more time than the 3 days we gave it, but if you only have one day there then the Extremo Canopy Tour (zip lining) is a must... and upgrading to going "Superman" on the last 1km stretch may be the best $5 you'll ever spend.
If you're scared of heights, this won't be fun for you, but I'm not crazy about heights and I had an incredible time. The guides are very helpful, friendly & have a very playful sense of humor (there were a couple of moments where the jokes were on us). One of our guides was a very cool guy named Danny, who will be running a bungee jump that Extremo will be adding to their list of offerings in the near future. If you see Danny tell him C. J. says "hi".
Overall, I can't imagine there is a better zip-lining tour in all of Costa Rica & if you think you have one that offers competition... please let me know.