There are four ways you can identify yourself to U.S. Customs and Border Protection:
Show your passport to enter to Mexico and come back to the US. It takes about 4 to 6 weeks to get a passport if you submit a regular application. If you expedite, you can get it in 2 to 3 weeks for an extra $60 charge.
Real ID or Enhanced Drivers License
Show your Real ID or an Enhanced Drivers License to cross the border. You obtain them from your State's Department of Motor Vehicles.
Canadian citizens from the following provinces British Columbia, Manitoba, Ontario, and Quebec can use Enhanced Drivers Licenses. Canadian citizens can present an EDL when entering the United States from Canada, Mexico, or the Caribbean through a land or sea port of entry.
Trusted Traveler Program
U.S. Military ID Card
US Military ID Card (for members of the U.S. armed official maritime business). Military personnel traveling under orders may present photo ID and orders. Family members must present a passport (with the exception of children 15 and younger arriving by land or sea.)