Passport. Had to fill out a form on the spot. Everyone said I needed a yellow fever vaccine (I spent $110 USD getting this in the states before the trip) and had to bring the little immunization booklet that they gave you. Too bad nobody ever even mentioned it and it absolutely wasn't necessary. the reason why they 'require' it is because you may have been traveling through the Peruvian Amazon beforehand and they don't want you to contract it and bring it with you into Bolivia, but I'd be surprised if anyone has been asked to see the paperwork. Still, if you can get it for cheap, do it just to cover your bases. But note that it hurts like a b----