after showing for years and multiple times, really i can bring a lot or a little depending on what i need.
when i show in agility, it depends a lot on if i can crate out of the car or inside. if i am crating out of the car, literally all i need is berlins leash (she can tug on that), and treats.
similarly, if i am showing rah and its a tiny show, i bring: articles, gloves, string cheese and db. he also has a real leash and a slip leash because i show on a slip, but i bring him into the ring for stays on a real leash.
that being sad, my car is permanently equipped for shows most of the time. i have bowls and water, 3-4 crates and dog beds in there (i dont heavily dog bed them at shows beacuse they will destroy them in frustration). i also have a copy of their rabies in the car, and my own bag normally has extra rah medications. i always have my chair, extra blankets, and soft crates that live in the car. and emergency tennis balls, a chuck-it, and frisbees
my training bag comes but normally lives in the car so its always there - has 3 db's (i actually always have extras), 2 bags of scent articles, extra gloves, extra collars and leashes (training/slip/heeling and 2 flexis), 2 go out toys for rah, and a few tugs (leather, jute, and riot stick). also extra clickers, a copy of the akc ob rules, a cheat sheet on the orders in the B classes, and my emergency: saline for my contacts, chapstick, and silver nitrate sticks for rah when he rubs his toenails down to nubs.
if i am keeping dogs inside - rah has his big crate and then blankets to cover him beacuse he's crate aggressive.
f i can, i actually keep berlin in an expen instead of a crate.
i dont water my dogs at the shows most of the time they get it after they show or at home, but my dogs are used to not having it (unless its summer/outside - they they get it). most of the time my dogs wont even drink when they are on the road.
if we are travelling further or overnighting, rah meds and dog food and dog toys/chews for the room, as well as electronic collars for exercise. i need toys in the room and chews to keep my dogs occupied because they are crazy. i dont crate in hotel rooms, unless i have a group of dogs that dont get along. and overnights mean i always bring a full medical bag, but mine is excessive ha ha.