A friend of mine has a backpack that's straight out of a super hero comic. You'd be like: "Shit we need a...", and she'd just rummage through it and pull some random thing out. She had multi-tools, multimeters, adapters, etc. She's a real-life super hero.
Maybe I'll write up a better list, but a few things off the top of my head: multitool/SAN, USB power bank, headlamp, snacks, water bottle, layers, hobby (I usually have drawing/watercolors and spinning/knitting with me), multiples of cables/headphones, first aid kit.
I've always carried jumper cables but awhile back had a dying battery and had to jump it a lot. I got one of the power bank battery jumpers and it is amazing. Haven't given away the cables yet but tempted to. Water/snacks/first aid/blankets in car at all times.