[chibill]

This sounds silly, but have at least a day's worth of food and maybe two days worth of water in your car. And also, something to hold gasoline. Never have gasoline in your vehicle, because the fumes from even a <5 gallon bucket, should they leak, can pressurize inside of your car. I've had multiple people report their cars being broken into, but in reality, the pressure created by the fumes from the gas canister they had inside of the car cracked or blew out their windows.

As a police officer, I'd also say, it's situationally dependent.

But, here's a few things that you should have:

Markings for night time (folding triangles work the best, flares are good but only burn for about 30 minutes. The little triangles can be seen just as well).
A flashlight, preferably a battery charged one. Keep the batteries separate from it. Another fun fact is that if you have batteries in a metal container, they can drain.
A portable AM/FM radio. Fun for the beach, and good for weather reports or just hanging around.

Water, but preferably, in cans. Plastic water bottles can melt and the microplastics can make you sick. Budweiser makes good canned water, and also there's the new Liquid Death. 1-2 16oz cans will keep you hydrated, provided you aren't moving a lot.

A change of clothes, complete set. Socks, shoes, the whole 9. Make sure to have a cold set and a winter set. Ask yourself "if my car broke down in this season, and someone couldn't get me for five hours, would I be okay?".

Another thing, is everyone is mentioning tools. Autozone and other auto parts stores sell a ratchet set that has everything you would need to conduct minor repairs for around 50 bucks or less. Buy one of those. And have a torque wrench in your car for when you change your tires. A used torque wrench that goes up to 200 foot pounds (you set your tires at about 90 for most tires) costs you about 40 bucks, if that. Torqueing a changed tire means you can drive further than the recommended 100 miles on it safely.

Having a some sort of wrench with a longer bar is also good, to get your flat off. Since my car (even with its aluminum rims) is torque speced to 100 foot pounds which is like almost impossible to take off even with a cross bar.