We'll be staying in Cancun for a couple days at the end of December (actually over New Year's, too) and would love to find some non-touristy restaurants. Although we'll be staying in the Hotel Zone at the JW Marriott, we're not into that scene particularly. I've read about Las Habichuelas in downtown which sounds more to our liking.
We'll also be taking some side trips away from Cancun (including a couple days in Chichen Itza) so any suggestions of other 'hidden treasures' would be much appreciated!

I'll second Las Habichuelas, wonderful service, unique and delicious dishes. Sit in the garden, very prett.
Even though it's located right in the hotel zone, Laguna Grill is also great. It's just so pretty, and all in all, a great restaurant. We went later in the evening, not a touristy place, IMO.
Fodors has more suggestions in their restaurant section.
Oscar y Lallos,(sp?) on the Mayan Riviera on the way south to Tulum, is a wonderful stop, too.

here are a couple. The first one is excellent and you can walk there from JW - just go to the street, turn right and go a bit and then cross over. Sit outside on the deck overlooking the lagoon and you must try the shrimp appetizer. This is a restaurant from Argentina. Here is link:
http://www.puertomaderocancun.com/

Downtown, easily reached by bus, is a good Mexican experience. It is popular with locals and turistas - it is very good:

http://www.laparrilla.com.mx/

Thanks for the tips - I can't wait!

The italian restaurant in the JW was excellent
Walk next door to the Argentinan restaurant in the Casa Magna and enjoy good lobster and prime steaks with a to die for sald bar, Good selection of wines in both.