I'd also recommend bringing an extra crop or pair of spurs or something you may need in a pinch
Bring your absolutely necessary grooming items to brush your horse before you show, I also recommend wearing sweats to the show since you may be getting up early and to put your show clothes in a plastic bag on a hanger to keep them in order.
Also a rag to wipe the horses mouth
Shapleys shine or show sheen and if you have a white horse green spot remover or baby powder. Definitely shoe studs and a chain, water hay, leadropes the tack and a seam ripper to remove braids
Bring a bucket for water and some hay for a haybag as well
Well you have your basics like saddle, saddle pad/s girth bridle you know tack and then I always have a first aid kit in my trailer whether that's for yourself or the horse and then I make sure I bring 2 of everything because something could break. I bring brushes and extra rubber bands. You don't necessarily have to bring everything bring everything but I bring most of my stuff. Oh and also a lungeline because I usually get to a horse show really early so I can take some time to lunge my horse in the warm up. Hope that helped