You picked an awesome place then with Manuel Antonio. Gorgeous place. You'll see the monkeys and sloths in the national park. nice beach to relax on and have people bring you fruity cocktails.
Since you're going to be headed out towards that area and want to do something like a zip line, there's a place called Turubari (
http://www.turubari.com/). You will want to do this!!
You're strapped in laying down, as if you're Superman, so it feels just like you're flying through the air. So so so awesome, and highly recommended.
It's rainy season, so our rivers are nice and full, so that means you have to go white water rafting!
http://www.riostropicales.com/ is an excellent tour company that I used. Time of your life!
Also in the same area is the crocodile tour -
http://www.crocodilerivertour.com/. Must see! You get to see crocs in their natural habitat, rather up close. the tour guide will get into the water with the crocs and give em chicken (cuz he's freakin crazy!).
Before I go any further, how long are you going to be here?