[Nutsinatin]

Water or at least a bottle, it's hot (hopefully) and you're working hard in a leather suit, they might well have drinking water on site but with bottle you've got it there in your hand, much more fun if you're not feeling dehydrated; Irn Bru is okay but water is better (the bubbles make me feel grim on the bike as well).

Tyre pump is also good as well as a gauge, for inflating after, or if you get a little too enthusiastic letting air out. Cable ties are good for holding about 50% of a bike together, not just brake levers, duct tape should hold about 45% of the other parts together! Spare fuses/plug, I've broken a tyre warmer plug stepping on it and had to change it between sessions before. Gloves incase you need to do any bike work.

Depending on how broken in your kit is plasters/tape, blisters can ruin the fun.