I keep pretty much all of the things mentioned above in our car. We travel frequently through the mountains. Following a motorway car crash, I've also moved our high vis bibs to the doors. So each door has a high vis vest in its pocket. It's the only sensible place to have it as you step out the car, when there may be no safe way of accessing the boot.
We tend to keep a pillow and blanket on the back seat, just for comfort, but would also be useful in an emergency. Sleeping bags, full sets of spare clothes, waterproofs, boots and winter coats in the boot.
I keep emergency cash, but was surprised recently that it was 'paper' notes - rather than the 'plastic' we use now. Must have been there a while! So, it's worth making sure cash is current.
I have a tool for cutting seatbelts and breaking and cutting glass, along with a hammer and shovel. Also, very low temperature windscreen washer fluid, as temperatures can be -10 and you go through a lot of it.
I have a good med kit, including burn ointment, inhalers, pain killers, antihistamines, regular medicines and spare glasses for everyone.
Food wise, I keep ship's biscuits; there's no risk of them being eaten other than in an emergency!
In addition to the main preps, I also have a a more routine emergency bag for unforeseen overnight stays - which includes the usual deodorant, toothbrush etc.
Our car has a false floor over the spare wheel (which should be a proper full size wheel, not a spacer), and I keep everything in that lower area, so the boot looks empty at a glance.