A lot of things. Something displaying your citizenship, some kind of old picture or state ID, social security card or something that displays your SS number such as a tax document, 2 forms of address verification like bills or insurance paperwork. https://www.flytucson.com/articles/mvd-urges-arizonans-to-consider-getting-voluntary-travel-i-d/
I just got mine. All you need is a bill or two pieces of mail in your name with the current address, birth certificate and picture Id. It was very smooth.
Thanks! Your answer is awaiting moderation.