While there are certain documents you can bring to enter and return from Mexico without having a passport, it is HIGHLY recommended to travel with a valid passport. This will reduce the possibility of any complications when crossing the border. When you are on your plane, you will receive immigration forms that you are required to fill out for customs. You need to keep the part of the form that they don’t take in order to re-enter the country. If you happen to lose this immigration form, you can purchase a new one at the airport for 550 pesos (~$30). Although it won’t break the bank, the time it takes to get this form is cumbersome and we have heard it costing people a missed flight. If you are driving into Mexico, make sure and stop at the border to get your immigration form.
