If you are a US citizen, you need nothing except for your passport. On the return flight, you will also be given a customs form to fill out, which you will give to customs upon arrival to the US.
In order to re-enter the U. S. you need to show your passport. If re-entering from Canada or Mexico, you can also use a federal passport card. Other documentation that can be used is listed on the Department of Homeland Security's website.
