Just back from Playa Del Carmen and Cozumel. Definetly go with pesos for resturants and shopping.It may be easier to get them there from an ATM in a bank. Do not use the free standing ATMs on the street. I got 18.67 to the dollar from Santander bank a couple blocks west of the ADO bus terminal.
A couple of good spots for lunch, Kaxapa Factory calle 10 north between 15th and 20th Ave. also La Cueva Del Chango, Calle 38 north just east of 5th Ave.