I recommend going to Xel Ha. When Steve and I went to Mexico last year we went there and it was amazing. You get dropped off in the morning and do as you please for the day until the bus comes back to get you. There is a ton to do including snorkeling, swimming with dolphins, lazing around in hammock city, and there is even an underwater walk (kind of like scuba diving).
My personal favorite thing was the lazy river. When you get on you're in fresh water and when you get off you're in salt water. Along the way there were places to stop to either cliff jump yourself or watch others take the plunge.
Fabulous place and we'll defiantly be heading there again next year with our gang.