Things to do in Cancun?

If you're only going for three days, I would concentrate on chilling on the beaches. However, I would definitely take a bus or taxi into Cancun proper to eat at some local-looking taquerias, purchase Mexican stuff at much lower prices, etc.. It's a lot more of a cultural experience than just hanging in the hotel zone. Also, you should consider spending an afternoon taking a bus tour inland to see the ruins at Chichen Itza.

Go to Tulum. Rent a car and go to their beaches. Visit the Mayan Ruins. Have spa treatments at the Maya Spa Wellness Center: http://www.maya-spa.com/

You will not want to go home! Yum!