This depends on the type of trip clients are planning to have in Mexico as well as the time of the year. The season, the climate of your destination, the activities you plan to participate in, and the duration of your trip will determine what you should pack.
Clients should not forget travel documents (including the necessary paperwork if clients are planning to drive) and clients should check before whether they need to apply for a visa. They should take the required vaccinations and be sure to have adequate travel insurance. Although US Dollars are accepted in some popular beach destinations (such as Cancun), in most places, you can only pay in Mexican pesos. So they should bring credit and debit cards, to withdraw cash from ATMs. We recommend bringing the travel itinerary, with booking confirmations.
Consider that not all places at all times in Mexico have hot weather, it can get pretty cold in certain regions in winter months so pack accordingly. This is especially the case in Northern regions and cities at high altitudes (such as Mexico City). If clients go to a beach destination, they should pack a bathing suit, a towel, sandals (or flip-flops), t-shirts and shorts. If they go to more traditional, colonial towns in inland Mexico, they should bring more conservative clothes with them. Wherever they go to Mexico, it´s always good to bring a sweater or jacket for the evenings (or air-conditioned spaces).
Check whether they need to bring any medications. Taking malaria pills is not necessary for traveling in Mexico, but malaria does occur in some rural areas. They may want to bring diarrhea tablets, aspirins, DEET-containing insect repellent, and sunblock.
Finally, a pocket version of a Spanish dictionary can be useful. We recommend to learn some Spanish before your trip; it can enrich the experience.