Definately too many nice hotels. If you can stay at any of the Royal Resorts, they are highly recommended. Downtown Cancun is fun, shops, bullfights, and LOTS of clubs. Hit up Coco Bongo, Dady Rock and DadyO's is you want to have a good time. Typically it's $30 for a wristband to drink all night long.
If you want tours...Look into Isla Mujeres, Playa Del Carmen and Cozumel. There are ruins in various places and much to see! I'd check with the hotel front desk for information on all the local attractions.