Set of legs, legbolts and levelers is something i need now and then, depending on location ofcourse.
Have a tapping device in the toolbox.
Lockbars tend to break now and then so keeping a spare is handy.
If someone rips off a loose lockbar, walks away and the glass slides out is not funny.
I stay away from the WPC lockbars on location and use System11 era ones since they are much sturdier.
Invest in a single key locksystem and spare a few locks and parts.
Make sure every game is secured with headbolts,not only quicklock, and stock a few for if a game comes in without them.

Powerchords will be needed once in a while.
I order different lenghts and select them when they find their place on location so they don't lay on the ground.

Some NTC's for if you are bringing a game that trips the fuse.
Optocouplers are not yet mentioned but especially the sender is breaking down alot.Receiver not so much.
Manual inside every game on location.
When ordering flipper parts for system11 and WPC make sure you label them and make sure they do not get mixed up.
They look very much alike, but are not and can screw up things big time when mixed up. This happens so often, at least here in Europe.
Ball plungers ,springs, thin sleeves, and rubbertips (when you hear a clonk during ball plunge contact replace asap) in the correct strengths and lengths.