Cancun is divided into 4 areas.
1. Commercial strip, much like Miami Beach. Great hotels, bars, beaches, and huge social life. Party City!
2. The City. A real Mexican community with Churches, great restaurants, schools etc. Not touristy, but genuine.
3. Nature. South of the strip, on the way to the ruins of the pyramids at Tulum, just past the beach city of Playa del Carmen, is the nature preserve of XEL-HA (pro..show-haaah). You can swim in 1/2 salt and 1/2 fresh water. Great relaxing day. Here you can also swim with Dolphins, (very romantic).
Underground rivers come to the surface in Cenotes. Great swimming, in natural fresh water springs, (bring mosquito repellent).
Next to the ruins of Tulum (hire a guide) is a fantastic beach. Don't miss it.
On your way back from Talum, don't miss the Buena Vida beach bar in North Akumal. Have a cold beer on the beach chairs or join with the locals sitting in swings at the bar.
4. The Jungle. A tiring, all day trip to Chitzen Itza (tourists call it "Chicken Pizza". A huge pyramid found in the jungle. If you can still climb to the top, it is scarry walking down.
Enjoy your time in Cancun. Don't spend all the time on the strip.