Zero Cruise Ships stop in Cancun...however, Cozumel (small, very cruise populated island) is about an hour or two away. In Cozumel, go to one of the "natural" waterparks. Very beautiful. If you are a history buff, visit one of the ruins of Mazaltan. The shopping on this island has dramatically deteriorated. No longer can you haggle and find good bargains. However, Kahlua (the coffee liquer native to Mexico) is a GREAT deal. Normally priced around 22 USD a litre, you can find it as cheap as 8 USD a litre. Lastly, if you are into the partying/good time scene, visit renowned Carlos n Charlies Bar or Senor Frogs Bar, both next to each other right off the pier. Have fun!
X-caret is a wonderful park at playa del carmen not too far away. I have been twice and would go again. Especially enjoyed swimming the underground river. Entry is about $50 but includes night show which is awesome.