Could not agree more. Shade, a seat, and some cold drinks go a long way for the humans. Ice in the cooler can also be helpful for the horses. If you invest in a good cooler you can pack it the night before and still have ice a day or two later. Also, on the human side, several pairs of dry socks and a hat. I can live with many things but wet feet are not on that list. My show trunk also has a live-in tube of sunscreen and bottle of insect repellent. The best trailer tack rooms have mirrors.
For the horse, the obvious things like jugs with water from home, buckets, hay nets, spare halter, etc etc. My favourite things for the horses are a saddle pad bag (ideally the ventilated ones - as the day goes on, this becomes your laundry bag. When you get back to the barn and your brain is fried it’s easy to remember that everything else goes in the barn, that goes to the house), a bridle bag with hooks (if you are an eventer, ideally three hooks - one hook per phase), and stud blanks (fussing around with stud holes at the event with an excited horse is the worst). A pen, paper, and safety pins are never a bad thing to have. And obviously, a generous stash of treats!