swimming with dolphins, jetskiing, visiting the mayan pyramids, bus to downtown and hit all the clubs - Senor frog, coco bongo, etc.
if you're staying at an all inclusive resort, for food, it's best to stick to the hotel food since you know their water/ice is safe.
When i went dolphin swimming we took a ferry to an island (mujeres or something?) that had everything you wanted to do shopping and water wise like snorkelling to see stingrays and sharks, and a pool etc. all of this was included with the dolphin package.
This island also has a lot of souvenir type places for everything from funky sombreros, cheap cigars and jewelry.
When you hit the clubs it might be a good idea to bargain with the vendors outside for unlimited drinks and all that (If you're with girls it might be easier), and apparently there are places where you can buy cheaper tickets for the clubs too, but I don't know about that cause we girls always got in for free or at a good discount
PS:
if you're gonna bus, don't give US dollars, give pesos if you can cuz it only costs 6 pesos i believe. Some hotel people told us to just give US dollars for bus fare, but after a while we realized we were getting ripped off giving like..10 US dollars each time when we should only be giving like, 6 dollars. They won't give you change and they'll accept it if you overpay.