Stay away from Cancun. It's really just a dirty sping break town. A little farther south is Playa de Carmen and Riviara Maya. Much nicer area. We like the RIU resorts. There's usually 3 or 4 right together and you can eat/drink at all of them. Also it is cheaper than Cabo San Lucas or Cozumel.
We got married in 2004 at Moon Palace resort just south of Cancun. It was so massive that we did not have to leave the grounds for ANYTHING. We are going back to Mexico to celebrate our 10 year anniversary. My wife found a phenomenal deal at a 4 1/2 star all inclusive on Cozumel Island. Wherever you end up I'm sure you will have a great time.