re: Manuel Antonio (hope I'm not too late)
The national park itself is a great place to see a lot of the native wildlife. There are boat tours you can take through the park.
If you have a car, there are quite a few places in the nearby area to visit. If you are interested in whitewater rafting, the Savegre River offers some great river rafting (
http://www.raftingsavegreriver.com/). A little bit further south is Hacienda Baru (
Home English - Hacienda Baru) which offers wildlife tours and ziplining through the canopy. If you are interested in ATV tours, there is a place in Uvita (
Welcome to Adventure Motorsports’ Jungle ATV - Jungle ATV) that takes you on 6+ hour tours up into the rainforest. When I went, we saw several waterfalls, one of them over 100m tall.