Ashley's right, so often you won't have just what you need, but there are some items I'd never be without.
A multimeter, a cheapo one will do. So often someone says my boat won't go, or it cuts out or the radios erratic. 99 times out of a hundred it's a duff or not charged battery.
Those little allen keys that fit huco and similar couplings, things become loose.( use threadlock to avoid this)
Superglue for fixing the mast, rigging etc you caught getting the model out of the car.
Oil for a quick fix of a dry motor, shaft etc.
Spare batteries…… See above.
Sware box, you'll make a fortune out of all the expletives when something unexpected goes wrong.
A magnet to find what you dropped at the pondside, and ancient eyesight can't find it.
Your sub's for the club, because you kept forgetting at the meetings😡
Spare boat to retrieve those that break down. Get someone else to take a spare too, to retrieve your broken rescue boat……. Don't laugh, I've seen it happen.
To avoid most of the above, maintain charge your batteries regularly, and test the model properly the night before, then remember to switch off. I've made that mistake too.
Chas