Most places take U.S. currency but it is a good idea to change some into Colones which you can do in the airport in San Jose, not sure about the airport in Liberia but I'd bet you can. Just pack your boards real well and they sh
as surfdog said, most places take USD, but you get a better exchange rate outside of san jose. also, you can get USD or Colones out of a lot of atms. go to kinkos and get some bubble wrap for your boards, if you care about them. the more padding the better. have fun, pura vida!
it's pretty much all been covered: yes, most places take USD, but it's best to have some colones on hand just in case.
most places rent boards, but if you're taking boards down there, pack them well. if you're taking more than 2 (no reason to, IMO, unless you're super hard on boards), it's really best to invest in a wheeled coffin bag. it makes life a LOT easier moving around airports & streets if your boardbag rolls instead of having to lug it around everywhere.
i'd also suggest that you pack light. boardbag & a backpack, tops. boards, fins, leashes, towels, etc...into the boardbag; everything else into the backpack. you can live (comfortably) for a month there on that set up.
when are you going? that will determine what type of boards you need to take.