I don't care for Cancun except for the clubs @ night. The beach would be amazing, but for the ridiculous tide/surf and how overbuilt it is. I'm more of a beach bumb than a clubber.
Try to get down to Tulum. See the ruins (bring lotsa H2O, it's awlays hot), go swimming, etc. Also might wanna try the park at Xpu-Ha. Theres a cove there and I think at Xel-Ha where you can do lotsa snorkeling, scuba, etc. I enjoyed that.
Puerto Aventuras is nearby. Much more low key with a small but really nice beach if that's your thing.
Playa del Carmen is cool for an afternoon until you get sick of the hawkers.
In general, stay away from tourist traps and try some local cuisine. Kills me that people go there and then eat at Hard Rock or whatever. What's the point?