2 gallons of water in gallon jugs. a couple water bottles on top of that isn't a bad idea. A couple hot cocoa packets, no problem tossing 4 MREs (in my case 8, but I have family) in with this.
2-4 space blankets (they are CHEAP and you may not be the only one stuck.) I keep 2 regular blankets in the car folded neatly and draped over the rear seats. 2-3 cheap pocket sized ponchos and a couple roll up windbreakers. Gloves! Add a basic alien abduction kit- one complete change of clothes, pocket toothbrush, talc, beach towel.
Don't skimp on the first aid kit here. bulking it up won't take up appreciable space in the car and you don't HAVE to pack it all out. You may end up being the one who has hemos even if it's the guy at the other end of the pileup who knows how to use them to close an artery.
Tools: pruning saw, axe, digger. That cna be an E-Tool, or a short spade, I find a wide trencher with a shortish handle works best. I prefer a decent axe- a collins youth felling axe works out well, but don't pop a 10 inch hatchet in there.
For the car itself:
reasonable auto repair tools, extra belt(s), tire valve tool, hand pump (yeah, it takes a while. but it works), make sure you have a real spare. Duct tape, a panel of visqueen. spare bulbs, fuses, oil, coolant, some bike inner tubes and RTV (trust me, I've fixed radiator hose leaks this way for limited periods of time), flares. Chains as appropriate.
sounds like a lot, but I could fit all this in a datsun 210 back in the day and since it mostly fit in void spaces or the way back of the trunk - and in the spare tire compartment- you didn't notice it. It's actually harder to pack all that into the Ford Windstar. because it has fewer void spaces.