I assume you are looking for suggestions off the resort property.
Go to trip planning, look at the tours section.
My 16 year old and I enjoyed the Tulum and Beyond tour in 2005.
My other daughter and I enjoyed the Sea to Cenote tour this year.
If you are into ruins, there are several options there.
Many people on this site recommend the Fat Cat catamaran tour .. again do a search to read about people's experiences.
Snorkeling at Akumal/Yal Ku Lagoon (do a search to get the particulars on how to get there).