I am surprised that many of you still recommend using a credit card to pay for your hotel stay, and use the ATM machines.
Your bank in the United States is taking a 2-4% charge on foreign currency exchange. So, for example if spend $300 on your hotel, $100 on other charges, and a $500 ATM withdraw, then you have paid the bank $18 to $36 for the privilege of partying in Costa Rica.
It can get even worse if the hotel quotes you a rate in USD because the hotel gets to decide the exchange rate when they actually bill you in colones.
You can use the credit card to book the reservation, however at check out time just pay in cash. Exchange your cash dollars at the local Costa Rican bank, especially Citibank or Banco de San Jose and you will get the best possible rate.
Of course all of this really requires a room with a safe, and avoid getting robbed on the way from the airport.