I have been twice, looking forward to the next time I can go.
Stayed at the Beach palace resort. The Palace resotrst have about 8 different places that you are allowed to go to and use any/all facilities/restaurants.
I liked the Tulum tour, and snorkeling at Isla Mujeres was nice too. Hear good things about ChizenItasa (SP) and I liked Xcaret which was an ecological park that has snorkeling, etc......