In general, don't go to Cancun for the quality of the dining. That said, two weeks ago I had one of my best Cancun meals ever at Labna (see Herb687's post), which is a sister restaurant, and right next to, La Habichuela (see work2fly's post). Taxi drivers know how to get there.

Both of these restaurants are half a block away from a park that is open well into the evening. There are food stands in the park that I haven't tried, but they look terrific.

Another great option is to buy an achiote chicken from the old lady who sells them in one of the downtown marketplaces. (I don't know how to describe her location other than she's across the street from a San Francisco supermarket, very close to the tortilla factory). She cooks these wonderful chickens using a very traditional Mexican spice -- achiote -- in an open air, wood buring oven, and sells them for about US$5 each with tortillas and hot peppers. The locals eat them on the spot (there's a single table with about three chairs); others of us bring them back to our hotel.