we like to just walk around and let our noses be our guide. I don't know that I've ever had a bad meal in Cancun. On our trip a couple weeks ago here are a few places we ate at and enjoyed:
Viva la Vaca - moderately priced, a argentine style beef (very tasty), as well as seafood, chicken, paella, etc (near convention center across from la fiesta store), small & friendly, consistently good. very casual
Casa Tequila - moderately priced, a wide choice of menu selections and a HUGE tequila menu. small & friendly. (next to viva la vaca) very casual. PS: XQ Grand Reserve Tequila ... ohhh myyyyy.
Puerto Madero - more expensive. Nice location, beautiful outdoor bar area, awesome steaks & seafood. they say they are casual, but we felt next time we'll dress a bit better.
La Ruina - cheap eats. Mexican and Yucatecan food. Consistiently good food. (plaza terramar, across from RUI)
other places we've like on past trips: Lorenzillo's - expensive. Love dining under the stars on the dock section. Not so fond of dining indoors. Lorito Joe's known for the all u can eat lobster or shrimp buffets (on friday thru sunday). La Distilleria - moderate to expensive (depending on how much tequila you try)

mexican food. In La Isla Mall (shopping works up an appetite), Casa Margarita, Mr. Papa's (cheap and lots), Starbucks for when you absolutely need that dopio, and Johnny Rockets for when you need that milkshake fix.
ok I'll stop now. I'm hungry.