The weather in Mexico is hot; it is not unheard of for cities like Mérida to see temperatures over 100 degrees even in April. Most of the popular coastal regions like Cancún and Acapulco have slightly cooler temperatures. Popular inland destinations, such as the Mayan ruins at Chichen Itza, also are extremely hot.
A basic list of clothing should include several pairs of shorts, a wrap skirt or skirt for women, several tops or T-shirts, a pair of casual pants for wearing in the city, a light jacket or sweater for nighttime, beach shoes and beach attire, including several swimsuits, a dress outfit for more formal dining, and several pairs of shoes, including sandals and, for going to the ruins, hiking shoes.
Sunscreen is a must in Mexico to protect from the sun's rays. Additionally, a light hat can be an invaluable addition to help stay cool. Bring bug spray, especially if you are headed away from the coasts. If you aren't staying in luxury hotels, a sheet sleep sack can be a nice way to ensure you have a clean sleeping experience. Most Mexicans speak English and Spanish, but an English to Spanish dictionary can be helpful at times, and, if you make the effort to learn some Spanish, it shows a respect for the culture.
A daypack can be useful for excursions away from the resort; many items of luggage can be purchases with a zip-off daypack.
A camera is a must so you can document your trip. Make sure you bring plenty of memory cards if your camera is digital or lots of film. A spare battery for your camera is a good idea, and don't forget the charger.
An MP3 player can help you to relax on the beach. It also is helpful to distract kids if you are taking a family trip.
As of June 2009, documentation requirements have changed. You will need a Mexican Tourism Card. If you are traveling by air to Mexico, you will need a valid passport to return to the United States. If you are traveling by land or sea, you will need either a passport, a PASS (People Access Security Service) card, or an Enhanced Driver's License. A PASS card is much cheaper than a passport, but a passport is great to have to travel anywhere.