I've never really taken in the Cancun scene...just not my type of travel tho many enjoy it. May I suggest a day trip over to Isla Mujeres. Don't take a tour group...get a cab ($10-20 depending on where you are for up to 4 people..negotiate with cab driver on the front end) to the 30 minute ferry over ($4-5/person each way). Have a cocktail/beer on the way over. When you land take the bay side street you arrive on north to Playa Norte (north beach...note it is topless in some areas)...wander in any number of the beach bars. Stop by the bar at Posada del Mar hotel for happy hour as you will run into a number of US expats/retirees that are very friendly...say hello to the bartenders Romy and miguel...super nice guys. Feel free to take a dip in the bar side pool.. Head east and wander up and down main street...lots of good bars and restaurants that are very reasonably priced ($5 for two margueritas/beers $2 each)...decent lunch/dinner can be had for under $6/$10 per person).
Go to the town square AND PEOPLE WATCH. If you get there early enough go to the market place and stop in at Hortensia's shop...pick out a fabric and she will make the wifey a sun dress you can pick up later in the day for $10-12....women dig it.
Very few cars on the island...cabs are $5 anywhere you wanna go...flat rate and will haul up to 4 people for that amount. Take a cab ride down to the south end of the island (only 5 miles long x 1/2 mile wide) and stop in at Hotel Garrafon (not the touristy amusement park...the little hotel next door to it)...snorkel on the reef...mask/fin rental about $10...good little inecpensive snack bar there. Or rent a golf cart for $30/half day or moped $25/half day and explore on your own.
For info..
IslaMujeres.info - Your Guide to Isla Mujeres Mexico