I was there... years ago!
We went to Xcaret (
http://www.xcaret.com/) it was a big park with lots to do. Like mini ruins, swimming with the dolphins, snorkeling, swimming, underground river, etc.
Anyway it is all fake but I thought it was pretty cool, I did the snorkeling while the rest of my family just sat the on beach - we also did the swim through the underground drive and swam with the dolphins (amazing!).
The one thing I liked about the xcaret was the area they had to do the swimming, was like right in the ocean but blocked off where as lots of other places we went to just did them like in a big swimming pool - I like actually being in the ocean.
We have been talking about going to mexico soon as well (like late summer or in the fall).