As of 2008, all travelers wishing to travel outside the United States by land, air or sea must carry their passport with them. Some travelers also carry a copy of their birth certificate with them as extra proof of citizenship; if you do this, make sure you take a copy, not the original. You should also take your driver's license--both to have a second photo ID with you and because you'll need it if you drive or rent a car. You may need your driver's license to charter a boat or other water-sports equipment. While it's not precisely documentation, you may want to carry a pocket-sized or electronic Spanish-English dictionary with you, if you think you may need it as a communication aid.
Plan to carry traveler's checks and cash on your Mexico vacation. The advantage of traveler's checks is that they can be replaced if lost or stolen; the cash advantage is that, obviously, it's accepted everywhere. If you bring your ATM card you can usually withdraw money directly from your bank account in pesos at the best exchange rate available, allowing you to keep less cash on your person. U.S. dollars are also often accepted as valid tinder, and are sometimes preferred; this depends on who you're dealing with, so keep a small stash of American cash on hand to use as tips or a haggling tool.
If you're participating in a guided adventure, there's a good chance that most major gear, especially those items that might be heavy or difficult to transport, will be provided for you. You should still check the guide company's packing list, however, as you may be responsible for bringing certain items. If it's not posted on their website, ask for one. You'll want to bring as much of your own footwear as possible for the sake of comfort and preventing blisters, with the possible exception of very bulky and heavy items like mountaineering boots. If you're taking an unguided trip on your own, make your own packing list and review it as you pack to make sure you're including all the necessary gear. Don't forget to pack sunscreen, too.
Average temperatures in Mexico can vary widely from 40 to over 80 degrees Fahrenheit depending on location and elevation. Inform yourself about what sort of temperatures to expect in the region you're visiting and come prepared with layers to suit. Also be aware that from June to mid-October you're likely to encounter much more rain than during other months; make sure you're prepared with raingear and that you take the climate into consideration when packing; for example, you may want to avoid using down products during very wet months, since they lose so much of their insulating value when wet.