Camera. Second most important to the horse
Maybe do a run through at home, like a dress rehearsal. Pack up like you are going to a show and then get ready for the "show ring" and ride, pack up and see what things would have been handy.
I am not going to write all the obvious like saddle, bridles etc but a few things that I find are handy are
Snacks for you and the horses
And that's all I can think for now :)