From personal experience, I’ll emphasize the fire extinguisher recommendation. Had it not been for a Good Samaritan trucker and his fire extinguisher on I-70 a number of years back, my car would’ve looked like a piece of burnt toast. Since then, all my vehicles carry an extinguisher.

Part of my after-purchase indoctrination with a new vehicle is to go through a “mock” tire change, just to familiarize myself with that vehicle’s jack and procedure. Better done in the comfort of the driveway at home, than at night/in a snowstorm/110 degree desert heat/driving rain.

Speaking of which, I carry a 12-in. x 12-in. piece of ¾-in. plywood in both cars. Provides a stable, even base on which to place the jack during a tire-change. Useful when you’re in snow, sand, mud, or uneven ground.

Fix-a-flat was mentioned. Myself, I carry Slime. I’ve never had to actually use it yet (thankfully), but it seems to get favorable reviews. YMMV.

I carry a spare/new serpentine belt. On my Jeep, I’m up-the-creek if the belt fails. The ~$30 cost of a spare belt is cheap insurance. Like the tire-change routine, I familiarized myself with belt installation (belt pattern and tensioner function), and carry the appropriate tools (ratchet/breaker bar).

I also carry a product with duct tape-like versatility: J-B Weld. It will handle a variety of makeshift/on-the-spot repairs that duct tape just can’t handle.

A small, but useful tool is a wire battery-brush.