This is what I carry. It is always evolving. Tell us your ideas of what you carry.

my spare box has
Set of belts
Set of radiator hoses
Various size heater hoses and clamps
Set of plugs
Leads
Disy cap and rotor button
One wheel bearing kit
Grease
Top ups of all fluids.
Fuel filter
Gasket goo
Various globes
One shocker
Service manual
Roll of 100mph tape
And roll of tie wire.

It did take a while to get together.
Not all of it brand new. For example the distributor cap is the old one from the last tune up and so on.