Just an ID. You can get back into the US (or whatever county you are from) with a passport. You can't get back in with just an ID. I always keep mine either locked in the room safe for the entire trip, or in a very good hiding place in my luggage if there is no safe.
It's a good idea to photocopy the picture page of your passport and keep the copy somewhere separate from your passport. Carry just an ID, leave your passport in your hotel safe. Be sure to save your FMT (Mexican Immigration form), you will need it to leave Mexico. It's best to just keep it with your passport. Technically you can get back into the US just by convincing an agent you are American. That process would be painful and would take hours of interrogation. It happens infrequently but regularly that someone shows up at a border without any documents and claims to be a citizen of that country. So if the agents believe you to be an American they will eventually let you in. But it's far easier on you if you just have your passport.
yep I agree, lock it up, have a photocopy back at home and in another place in your luggage somewhere just in case. plus, I wanted to post a comment so I could see my counter
