We went here to swim with turtles, and it was amazingly good fun. Suitable for all ages that can swim. They also have deep sea diving.
This cenote is right across from the hotel, but there are dozens of other ones in Riviera Maya.
Tulum is only a 20 minute cab ride away and in addition to gorgeous beaches and the Mayan ruins, there are cute boutique hotels and great restaurants.
We haven't done this yet, but REALLY want to do it. May is one of the best times to see the whale sharks.