I remember a thread, in GP forum I think, were there were lots of posts on what to do. Do a search.
I was there 3 mos ago on an Award. Picked up a rental car and drove to the northern coast. I got a nice all inclusive hotel from the Travel Auction site - and just vegged out much of the time. Roads are a bit rough, but not too much traffic once outside SJO (you do not need to go through the town).
Also went to Jaco area (Marriott). It is very touristy and I do not think beaches are as nice. But then I did not think any of beaches I saw were up to Hawaii or Mexico standards.
There is an ATM in Airport near the TACA help desk and airport info desk by, I think, Gate 4. So you can get some colones before heading out - although many places take US $$.