I am a worry wart so here's my short list: emergency car kit in case I have a low tire; a 30 foot cotton lead/lung rope; a spare halter and lead rope; coggins; flashlight in case I end up arriving or needing to check tires in the dark. I'm a little paranoid about tires. My GPS plus a real map in case there are detours.
Before I hauled the first time I practiced backing and parking; and before I hauled the horse the first time we went for a short trip just off property and back again so that his first ride wasn't a long one.
I use shipping boots for the horses that walk on themselves, but not for the others. I also tie when I'm hauling 2 but never with a rope halter - always a reinforced nylon halter.