(1) A nursing license valid in the state you are going to be working in. Without this you will be the most overqualified janitor at your hospital until you get your license.
(2) Any Certifications you may have in the field you want to work in. Certifications may not be required to work in your field at your current hospital, but many travel nurse agencies prefer that you get certified in your field before starting an assignment. I think this is so the travel nurse agency can show they properly vet their nurses.
(3) Your Basic Care Life Support card.
(4) Your Immunizations.
(5) Driver's License (or ID Card) and Social Security Card.
(6) References. It is a good idea to get a written reference from a supervisor at each of your past hospitals. I tend to collect them. If one of my supervisors is particularly pleased with my work I will ask them to put it in writing. That way I can take it with me and use it for my next assignment.