Duct tape... good for patching a worn hose or silencing a backseat driver.
When I was in college, I used to keep several rolls of athletic tape in the car. I always said it was the most useful stuff on earth.
I now carry duck tape. Even better.
Agree on the duct tape. Not a fan of it for “permanent” repairs, but it can be a great temporary fix to get you to a better place.
Also big fan of tire plug kits and a portable compressor. I went through a spell where I picked up nails regularly. I assume someone hauling construction scrap was using the same route to work as me. Anyhow, it got to the point that I could plug a tire and reinflate it faster than I could have hauled out the jack and spare and changed it. Keep a garbage bag or some cardboard so you can lay on the ground if you’re in nice clothes or it’s raining.
It’s also wise to be familiar with your vehicle. Plenty of message boards out there that are car-specific. Find out what is a common problem and either fix it before it breaks or have the means to fix it on the go.
I’ve learned to hate in-tank fuel pumps. They generally go without warning, and then you’re stranded. Worse, you can’t fix them without dropping the tank. On every pickup I own, I cut a hole in the bed and put in a marine hatch right above the fuel tank access. Then, with a spare fuel pump, I at least have a chance to get myself back on the road if/when it dies, and I can change them out regularly with some ease. Not always practical to do in cars unfortunately as the tank is generally under the back seat.
Wire, for those times your exhaust system starts sagging, etc...
Zip ties just because...
Cannister of wet wipes
Up north I always used a hatchet for breaking ice ; always kept it within reach for busting hands if anyone tried to get into the truck, but it also would work to put deer,etc., out of their misery if they were struck by a car.
Teflon spray for frozen locks.
Coil of rope and corn or dried apples for stray livestock. Dried apples make a pretty good snack, too.