I have a TON of stuff in there because it functions as my regular tack room right now, but make sure you have first aid and at least a few basic grooming items. It’s a shame to get somewhere and need either of those things. If you have a spare girth, keep it in there. I’ve known so many people to load up their saddle and tack, tack up at their destination, and discover they have no girth!
For organization, I have a gridwall and baskets I can hang. I bought mine through the organized barn. While the product is great, they are priced higher than other places and frankly the shipping/customer service sucked. There are other places you can source a wire rack with hanging baskets. Check out organizedbarn.com and http://horsetraileraccessorystore.com/ for ideas of things you might need/want. Also, a trailer aid for changing tires is very helpful!