Here are a couple of my experiences of my past 4 trips there. I'm going back in 6 weeks, woohoo!
The bullfight in old Cancun (10-15 minutes by bus) on Wednesdays are cool. Couldn't make my wife go with me though.
Club CocoBongo is fun to party at, then of course you have Senor Frogs, Carlos & Charlies, and Fat Tuesdays. The Hard Rock Cafe sometimes has live bands. Those bracelets you buy for entry ($25) and free drinks are worth it, but only after you tip the bartender to keep hooking you up, sometimes they're bastards and want more and more.
Had fun on the Captain Hook Pirate ship ride, the food quality has been going down a bit, but the show is fun and afterwards you still have time to party.
Stay away from "american" restaurants, they gouge you... stick w/ tacos and beer!
Oh, and if you go shopping, haggle haggle haggle... they can ALWAYS go lower on whatever you're looking to buy. Mercado 23 & 28 in old Cancun are great for picking up stuff to take back home.
The Chichen-Itza pyramids are cool the first time, but let me warn you, it's a loooooong ride there and back, something like 2 or 3 hours each way... and HOT!
The eco parks (Xel-Ha, Xcaret) are nice, you get to see alot of native species and even swim w/ dolphins.
The scuba diving, well, depends on what your used to, alot of the outifts there cater to beginners and you won't have fun if you've done it before or are experienced. If you decide to dive, ask around and check for recommendations for a place that will take you out to the real reefs.
Isla Mujeres is great for shopping, and the double decker boats that take you there usually have a band and free rum punch, or as they say it, "ronpunch".
Lots of other stuff to do, some you'll have to experience yourself, Have Fun!