vehicle-specific tools — a BC fire extinguisher, work gloves, tire pressure gauge, jumper cables, full-size tire iron, a decent portable jack (a bottle jack?), portable jump-starter that's able to charge cellphones
2-3 gallons of water (When in freezing climates, I like to store plastic water bottles inside an insulated cooler — the cooler is more for physical protection while bouncing around in the trunk, but it can reduce the amount of time that the bottles are frozen as well. As long as the water bottles are small and flexible enough, freezing shouldn't damage them too much.)