I've been to 3 all-inclusive resorts on the Yucatan Coast south of Cancun. They were all great vacations. We have two teenagers, and I felt very comfortable letting them roam the resorts together while my husband and I got some sun! The resorts are very clean and safe for families with children. If you're looking to find some places for adults only in that region, try the Adventura Spa/Palace, the Royal Hideaway, and the Riu Palace. The first one is adults only; the other two have very few kids (I think).
Our experiences were at the Iberostar Tucan, the Iberostar Paraiso, and the Barcelo Maya. Of those three, we liked the Iberostar Paraiso the best, although all were very nice, and each had their advantages and disadvantages.
The nice part about a vacation to this part of the world is that you can personalize your vacation - you can make it as busy or as relaxing as you like.
White sand beaches, turquoise water, the most difficult questions of the day are: "Do I lay on my stomach, or on my back?" and "Do I want a margarita, or a pina colada?" When do we leave?!?!?