Lots of smaller mom and pop places don't take credit cards. Places that do may well apply a surcharge. The easiest thing I find is to withdraw money from an ATM using my Canadian bank's debit card. Follow DandyDon's instructions above.
Lots of place do take CC though. My Southwest Chase visa pays in pesos with NO extra fees and a great exchange rate. I use it for EVERYTHING. I need those SW points.
And how many of you drank too much and fell down those terrible circular stairs at old Prima? I think they took out a LOT of people! Or current Caribe Burger?