A German visiting the US wants to visit Mexico for the day, what documentation do they need to return to the US ?
Assuming you are visiting the US on the VWP (Visa Waiver Program) and returning from Mexico by land you need only your passport and preferably your I94, which you were given when you entered the US. Even if you somehow don't have that something documenting your entry to the US should do, as you can apply for another. Even without that they will probably let you in. Your return ticket might be helpful. ESTA is irrelevant if you enter the US by land.