mlsmn, ... "Thai" at La Isla is one of the best Thai restaurants I have ever been to. Better than anything else in the US. I can't wait to get back there this February.
Pass on the "authentic" Mexican restaurant at La Isla. The food is heavy. There are costumed dancers and a show that takes to long, keeps you from having any discussion and isn't that good. Can't remember the restaurant name but it is near the center of the La Isla shopping center.
For a taste of America, try Chile's or Starbucks or McDonald's or Haagen Daaz (I think). All are across the street at La Isla.
The Melia (2 doors away toward Downtown) has 2 good, but rather expensive restaurants (one Asian; one with a Spanish, not Mexican, flavor). The Melia Hotel, in and of itself, is a trip. Very modern and Euro-Chic. Few Americans stay there.
Ask also at the Langunamar concierge desk. It had a list of "approved" restaurants last time I was there. None that I went to were awful. Many are right on the bus line that stops right outside Lagunamar's door. The bus runs often; I felt quite safe; and it very, very cheap to ride.
Cancun has many, many restaurants in the "Hotel Zone." Food is generally not a problem in Cancun - but I still drank bottled water or soda drinks outside of Lagunamar. Overall, food is a bargain in Cancun.
Enjoy! ... eom