You will need some type of travel document to enter Mexico. There are several types that are valid and any one of the following three is acceptable:

1. A valid passport
2. Birth Certificate with raised embossed seal
3. A voter registration card plus a photo identification such as a valid drivers license.
4. If you are traveling to Mexico with a minor, under 18, who is not your biological child, you will need NOTORIZED authorization from both of the child parents and/or legal guardians in order to bring that child with you to Mexico. (for example, your taking the grandkids, neices & nephews, your kids best friend, etc.)

See the Mexican Consular Info sheet, linked above if you missed this bit of info. As the sheet will tell you, this is needed to protect children from international child abduction.

http://travel.state.gov/travel/mexico.html

Once you have actually booked your trip outside USA or Canada, go here to register your trip with the US Government. It is totally free, takes only a few moments, and in the event of a natural disaster, act of war, terrorist attack, etc. they will know exactly where you are an make provisions for getting you back home safely and if you are a victim, they will be able to notify your next of kin expeditiously.

US Citizens go here:

https://travelregistration.state.gov/ibrs/home.asp

Canadian Citizens go here:

http://www.voyage.gc.ca/main/sos/rocapage-en.asp