Bring along a good pair of broken-in walking shoes, because uncomfortable feet will make for an unpleasant journey. Unless you like traveling with a lot of luggage, it is best to choose one pair of leather shoes that are both very functional and that can also function as stylish footware for casual functions.
Weather here is very different from that in the United States. The climate is hotter, and when the rains come they often come in torrents, especially along the Gulf Coast and the Caribbean. So find a sturdy umbrella that fits into your suitcase and leave the raincoat behind.
This is a Spanish-speaking country, though English is widely understood within the tourist industry. Still, some basic knowledge of Spanish will go a long ways to having an enjoyable visit. Bring along a phrasebook or a dictionary to help get an understanding of all the signs and place names. This is only the very beginning step in learning and understanding the Spanish language.
Mexico has some of the world's best beaches, but don't forget you will need a change of clothes to enjoy the evening hours. For men, a nice pair of slacks and a button-down shirt would work fine, while the ladies should at least bring a comfortable skirt and blouse and perhaps a nice casual dress.
When the sun shines in Mexico it almost always intense, so bring appropriate headgear and a good pair of sunglasses. Some sunscreen would not be a bad idea either, for Mexico is a very sunny place. Be especially mindful of the sun if you are traveling from a northern latitude to southern Mexico for a winter vacation. The change in climate can be very sudden at this time of year.
Shopping deals are plentiful in Mexico and if you enjoy looking for and purchasing unique clothing, then you might need an extra suitcase. The country abounds with such wonderful items as handmade hats, attractive shirts and leather sandals.