Good stuff to keep in your car is a great thing i am curious about. In alaska, I kept a bag of clothes (for anything, just handy as hell), 2 sleeping bags (one of them was a -50F bag; i have had to take naps on the side of the road in -30F before, without a sleeping bag it sucks), cigarette adapter tire inflator (i don't like those cans of tire inflator with goo, great for unbalancing your tires, and would probably explode in certain temperatures), first aid kit, a few mre's, cell phone charger, jumper cables, 2 quarts of my favorite oil, tire iron, jack, flares, a few pieces of board (whip out the jack, get some boarding under the offending tire, remove jack, and get yourself back on the road), and a full size spare (full size spares are easy to come by, get another tire on a rim to fit your car and put on your car, replacing your most bald offending tire; put this tire it into the trunk...beats a donut any day) I try to be prepared for the worst. As far as napping on the side of the road goes, i stopped buying cars with way too comfy seats. Comfy seats can easily equal into falling asleep behind the wheel. Unfortunately as minor discomfort will keep me awake, i don't have access to real non-car sleeping arrangements for when i do wish to sleep in a house.
What other things are good to keep in a car?
EDIT: also baby wipes. Those are great for cleaning off your hands, but more importantly for when you and the willing guest break in your car.