Cancun is extremely commercial and resorty, I'd recommend getting out of the city, for a few days if possible. Tulum is about 2 hours south, and is a nice little town that is close to a lot of good stuff. There are, of course, plenty of Mayan ruins around. The Tulum ruins are great, though small, you can see them in about an hour. The ruins at Coba are more extensive and very nice. Chichen Itza is the most famous, and is always PACKED, but worth it.

Other stuff: there's a nature reserve south of Tulum where you can rent kayaks and visit a couple of small islands that are home to a number of amazing birds.

One of the things the area is known for are so-called "cenotes", which are sinkholes that are often connected to cave river systems. There are lots of public cenotes where you can go snorkeling and see some interesting wildlife, but if you can arrange a private cenote tour that takes you on an adventure through the caves, that's the way to go.

Oh, and hit up some taquerias ;)

A two hour drive south is incredibly worthwhile. You can hit the Mayan ruins at Tulum (worth about 1-2 hr) and then head to Xel Ha, which is an outstanding combination of an all-inclusive (food, alcohol, snorkeling equipment) resort and an aquatic conservation park. At Xel Ha you can swim with tropical fish while snorkeling in a warm lagoon then relax with a few drinks and some decent food.