I emptied my trunk when cell phones became cheap. The stuff in my pickup won’t fix anything. The two sets of jumper cables came in handy when I had to clamp them together to start a car that was nosed into a garage.
Inverter and extension cord for extended Wifi browsing with laptop.
Ten amp battery charger to use from AC mains or combine with above to act as jumper cables.
Homemade lighter plug adapter to trickle charge battery with above.
Squegee.
Empty wide-mouth juice/sports beverage bottle, for when there’s no rest room around, or for rapid de-icing of windshield (the Air Force used to use a urea-based de-icing compound).
Jumper cables, the toolkit the car came with (a couple small wrenches, screwdriver with flat, Phillips and two sizes of Torx bits, lug wrench, jack), some more tools (pliers, small socket set, gloves etc.), a full-size spare (I ditched the donut when I got a set of alloy rims from a junked car), electric air pump, real lug wrench (the one it came with is rubbish; I have a cross-type that actually lets me take off the lugnuts!), bungee cords, light cordage for emergency tie-downs, box containing spare bulbs for almost every location on the car, warning triangle, reusable grocery bags, clever device to keep loaded grocery bags upright when driving home from the store, and a whole lot of Velcro to keep the emergency stuff in place.
What else SHOULD I have in there? A better tyre pressure gauge, locking pliers, duct tape, flares, collapsible gas container. Quite probably some emergency food and water, potentially blankets. I’m lucky enough to live in a very mild climate; it doesn’t snow here. If I ever went someplace it did, I’d add chains, blankets, ice scraper, and some means of heating water.