All you really need is a headlamp, a mixer and your instruments. A headset or a small set of monitors is helpful, but I try to have run through what I’m playing at least a couple times so I can be prepared to improvise during the inevitable periods when you can’t hear anything except the audience.
Someone mentioned earlier that if you have the option, get your levels right before you play. There will be many situations where for whatever reason you can’t, but it makes things go a lot smoother if you can. If they are cool with it, do it.
Just remember to have fun. You’re being asked to play for people…they want you there. You’re going to make plenty of mistakes, but 9/10 times, you’re the only one who notices. People are there to have a good time, so if you’re having a good time, you’re setting the right vibe.

EDIT PS:
Plug in with cables unless you have plenty of time beforehand to set up and test a DI box. I have had DI boxes take a dump before because we got in a hurry and it wasn’t pretty.