How long are you there for? Not really my idea of Mexico but you should at least see Tulum and Chichen Itza. If you have some kind of tourist coddled trip I'd break away a little and make that a must. That entire area around Cancun is way to commercialized and plastic for me but for a place to hang on the beach while surrounded by those doing the same thing is what you are after I'm sure its great. Was in the area several times many years ago when the bubble was about to burst. My brother goes to Isla Mujeres every 8 months and just stays there and loves it. If you have any kind of a stopover in Mexico city and the time, I'd most definitely go to Teotihuacan.