Well, first you'll have to dust off that passport. Otherwise, you probably won't make it out of the airport. I'll be honest, that's probably the worst part of a trip to Mexico. It seems like they make their airports as unhospitable as possible, just to make sure you enjoy what's outside a little more.
Next, you'll probably want to have a little cash on hand. In Mexico, a little green goes a long way. And don't expect to use the credit card at many places other than banks and bigger stores. Throughout much of the country, alley markets and sidewalk vendors sell a lot of the crap that people love to bring back to the states and proudly display all over their house. Just carry a little cash, because when you see those hand-made goodies, you're gonna want some.
(above)This image gives a nice layout of where Tulum is placed in Mexico with respect to the rest of the country.
(below) If you get a chance to wander away from the tourist section of Cancun, the real city is home to a bull fighting arena. It's one of only a few places in the world where you can see this happen