Hmm I’m wondering how the other posters can forget the absolutely most important thing: Gaffa Tape
SCNR
Rest depends on what you are doing at your “gigs”. DJ, live music, talkers?
Some cheap spare Microphones with stands and Cables are a good idea on any live event. For me, the T.Bone MB60 does a great Job in that: 3 pieces including clamps and a case for 35€ - you won’t find much cheaper ones. That minimizes your pain if you need some “Beer proof” Karaoke-Device, a moderation mic for Outdoor Action on rainy days or on any other opportunity where it might get lost/stolen/forgotten/fall down/damaged. Despite that cases it is an absolutely fairly usable SM58-lookalike which won’t do wonders but a fair Job on any application where you would want a dynamic cardioid mic - singing, talking, Toms, snare drum, most acoustic instruments, even harmonica or in Front of keyboard or gutar combos work. Surel not perfect by far, but absolutely better than nothing (and considering the price more than good). Don’t forget Mic stands and cheap small clamps to mount a Mic on things like a score or drum stand. And Cables. You won’t ever have too much of them.
Additionally, some cheap wireless 2.4 GHz XLR transmitter/receiver-Pairs also have been handy. Got them from ebay for roughly 50€ per Pair, they simply “replace” any analog XLR signal Cable (mic or line level possible) with about 5ms latency. Good thing to attach some additional Backstage speakers or mics where cables would really be in the way (or you run out of XLR cables). if you need to replace a broken XLR cable very fast or as a wireless solution for cheap dynamic mics. Same thing as the Mics: not a really professional top-of-class Tool, but cheap and good enough in case of emergency, at least much better than nothing.
If you work with USB-C powered devices, you also definitely want to have some cheap and freshly charged power banks in your bag for the case something badly needs repowering. This is much simpler and more usable than a charging station since you can bring the power fast whereever the device needs it, even on stage.
Cables: definitely you should a bag with a fair collection of all possible and impossible Adapters. Even CECon to Gardena would be useful some time 
Oh, and of course the electro emergency toolbox: Screwdrivers, Leatherman, Pliers to cut and crimp the most important types of wires and connectors, soldering iron, unsoldering pump, isolation tape, super glue, cleaning material, a simple multimeter and cable tester. Don’t forget something usable as a 3rd hand.
Some small wound plaster strips for the case you get small bleeding cuts on fingertips or such things are a good Idea, too. A small injurance happens fast and blood drops on Mics or all sort of paper don’t look nice (on the technician’s outfit they might bring some respect, though…). Use gloves whenever necessary, of course.
Always have some drinking water with you. You won’t perform a good job if you get dehydrated and if you are alone and in a hurry it can happen very easily to forget drinking and catering might not work for you. Water will work and can also be used for cleaning or cooling things and fingers. Some sweets for fast repower-up your brain are a good idea, too - no chocolate (makes too much dirt when getting warm), some cookies, gums or sugar-containing drops will do the job.
