One thing DH started putting in ours was a nail clippers, and I can't tell you how many times it has come in handy.
A bottle of deodorant for when I've forgotten to put it on in the morning.
Long charging cables for everyone's phones (for long drives).
The kids have fleecy blankets in the back all year, they love them even in summer, inexplicably.
A pile of face masks.
In winter there'll be a scraper and bottle of de-icer (though I might try a less nasty solution than de-icer this year, it's foul stuff).
We mainly drive within 15km of home on small country roads, so traffic jams and being stranded aren't really possibilities. We got stuck in snow before and just walked home, it wasn't a problem.
Good thread, though, it's very satisfying making sure the car is tidy but ready for anything 