My post from another thread on CR....
Go to the west side (Quepos), and hit Manuel Antonio National Park.
If you go to the park, spend $30 and hire a guide. He will point out a bunch of wildlife you likely wouldn't see. Most take spotting scopes with them so you can get a good look. We saw a couple sloths, monkeys, birds and lots of interesting insects. Outside of the monkeys, there's no way I would've found those animals myself.
There are a few beaches in Quepos...Playa Espadilla is the common, public beach that's right in front of all the bars. You can walk to a bar, pay $1.50 for a beer, once you're done go back and they'll give you $.60 back.
However the beaches in the park (Playa Espadilla Sur, Playa Manuel Antonio) are absolutely immaculate. You can't drink in the park though, so it's pretty nice to just go relax away from the busier beaches.