Costa Rica is a fabulous country. DO NOT get off the airplane and just go sit at the beach for 10 days. The Monteverde/Cloud Forest is an absolute must. There are several tours very close to Monteverde, including one in the National Forest and one hiking tour that starts in daylight hours and concludes after dark. The purpose of the hike is so you can see the daytime animals then see the nocturnal animals as they come out. The Arenal volcano is also a must. There are also several hikes/tours in that area. My wife and 2 kids and I took a guided boat tour (just the four of us) near the Nicaraguan border up river. We were literally by ourselves. More wildlife than you can ever possibly imagine. Took a horseback ride to a private waterfall. It is an awesome, clean country. One more suggestion--fly to Liberia and not to San Jose. San Jose is the only place in the country when we felt unsafe.
Went to the Osa Peninsula and it was an incredible trip. Saw tons of Scarlet Macaws, parrots, toucans, sloths, monkeys, etc. Quepos/Manuel Antonio was almost as good, was more luxurious, and had fewer bugs. Didn't care much for Arenal as there was way too much fog to see anything as I hear is often the case. The poster above liked Monteverde. I thought it was okay. Tortuguero on the Caribbean side was good as well. Some great things to do besides wildlife watching are rafting and deep sea fishing. I thought it was much cheaper to fly into San Jose. I don't think the city has much to offer though. So if you fly in there, spend as little time as possible.
