Besides all the above, which except for the libations and the larger items, like an instrument stand, I actually carry in a tackle box that serves as my gig box: If your amp is a tube amp, extra tubes. Extra fuses. A soldering iron, solder, jumper wire, assorted pliers (at least a pair of slipjoint and needlenosed) and wire cutters/strippers to hold what you're rewiring to keep the gig going. The $5.00 polarity checker so you don't electrocute yourself on a ground that turns out to be a hot. Not just one, but assorted screwdrivers to get into what needs fixing in the middle of a gig. Extra guitar strap. Velcro cord straps to keep it all from deteriorating into a mess of spaghetti. 3-to-2 plug adaptors (not for bypassing a needed ground, like in an old joint that as pre-code 2-prong sockets, but for lifting the ground if needed to something in the signal chain causing a ground loop). Capo (Yes, capo. I've got a dance band version of Boot Scootin' Boogie that I have to play in my dance band on request for a 2-stepper, and I'll be damned if I'm going to play it in Eb.) FX jumpers. I was going to say Gatorade, but the banana will cover the potassium loss. Corkscrew (you know, goes with the rubbers!). Multimeter with both probes and allegator clips to diagnose a crash in the middle of a gig. Heavy guitar slide (when it absolutely has to sound fretless, and you don't have one). Spare prenotched nut and bridge saddles/screws (yes, accidents do happen). I've even been known to carry a spare pickup when gigging heavily. Fine sandpaper. One regular file and a set of rattail files (A friend kept breaking strings every 5th song or so. I burnished his bridge saddles at break and he never broke another string) Spare knobs for both the bass & the amp. On the newer amps, spare modular power cords (yes, I've had them fry). Keep the old set of strings. If one breaks, there is not as much tonal difference between a string that has bedded in and a dead string than there is between a new string that will twang out compared to the bedded in strings. A spare of the FX you use most. Acetaminophen.
And look carefully at the backlines of the guys who are on the road for a living: they even have spare instruments and spare amps, so if one goes down, a roadie or tech can immediately switch over and keep the gig going, even in the middle of a song, especially if there are any sync tracks for lights, visual or audio effects, etc. But I've never played a gig that big.