Camera and film (If you have a digital camera and a
laptop computer why not bring it and e-mail your friends and family your
photos as you take them)
Good walking shoes and sandals - Bring sandals that
are made for the water. You can walk along the shores cooling off
your feet and have your feet protected at the same time. That sand
gets hot!
Travel information about your Mexico destination.
A travel guide and
maps will help you get around with less confusion.
Spanish/English dictionary - could get you some good
deals if you're up on the lingo at the markets when bartering.
Also shows you have respect for Mexican culture.