If I know it's going to be a longer/full-day ride, I will pack something for lunch/snack, a couple of bottles of water, knife, cell phone, camera, some little first-aid and anti-bug items for myself and the horse, extra bits of leather in case I need to make an emergency tack repair. I may or may not bring the horse-collar and lead rope with me. It's nice to have along if we take some extended breaks. I can just put the collar on my horse when I tie him and take the bridle off entirely, so he is free to scratch his head without the worry of him rubbing the bridle on a tree or tie-post and breaking it.
Otherwise, I like to pack light.
"Parelli horsemanship is just like painting by the numbers. You need absolutely no skill. You just put this color here and this color there, and when you're done, you have ... a mess no one wants." mp