My suggestion:
Take a flight to cancun. Don't stay there cause it's for tourist *****es, but catch a bus to the ferry and take that to Isla de Mujeres. Stay there for a week. The hotels are cheap, the beaches are beautiful. Great nightclubs and food, and all the pot you can smoke on the cheap. After about a week, take the ferry back and jump on a bus to Tulum. The entire place is full of resorts made up of people from all other countries that came and just didn't want to go back. The beaches are amazing and some of the best food you'll ever have. Go to the ruins and dive (if you do) in the sinkholes during the day while sipping back the tequila all night long (bring bug protectant, the mosquitoes are bad but only during dusk). On your last two days, go back to cancun and do any shoping you have to do and check out a few clubs. If you go remember to exchange your money rather than pay US currency (which you can do just about everywhere) and you'll save about 7% on what you buy.