While this is certainly not a complete list, it's perhaps a useful start.
I decided to just dump out my tool bag (a Shure microphone, zipper bag) and see what was in there.
* individual strings, by gauge
* string cutters
* needle nose pliers - to bend the ends of the strings, near the capstan, so they don't prick your fingers
* a nut driver - because the nut on the jack of electric guitars often comes loose
* a toothpick - I have strap locks on my Fender Stratocaster and sometimes the wood near the screws wears thin and the screws start backing out. So, you stick a toothpick down into the hole and break it off. That gives the screw new wood to grab onto. You can repeat this trick, over and over.
* a string winder
* extra picks
* a cloth for wiping down the guitar (honestly, I don't use this very often)
* hex drivers for truss rod adjustments or locking tremolo systems
* assorted screwdrivers - one even has a magnet on the end, for grabbing small screws that fall into odd places, as small screws often try to do.
* a pen (paper not in the picture) - for catching all those genius song ideas and any notes you might make about gigs, contact info of musicians (and maybe groupies, too)
There you go. Have fun, you wacky rock n roll boyscout, you.