If you just want to enter Mexico, ya dont need jack shit..
however if ya like America and wish to come back have a passport on hand.
Just in case they do want identification....
Most of the time if you are white and looking normal they just wave ya through after a few questions.
There's some sort of a Special Zone within some number of miles of the border and encompassing all of Baja California for which you need only your Drivers License. They really don't check that sort of thing at the border in my experience, been through and back at least 8 times and never even been searched. All they did was ask our citizenships. Security might be tougher nowadays.