My car has a handy dandy secret compartment that can store all sorts of things. I've filled it up with supplies.
When you put this together, consider the needs of your family. If you're active, like us, you'll want things like sunscreen and bug spray and moleskin for blisters. These are practical items you may use regularly (just remember to replace them for next time). I also keep a six pack of bottled water. I know that you aren't supposed to keep bottled water in a car because of BPA concerns, but in a situation where your car broke down and it was 90 degrees and you had to hike down a mountain, that bottled water will be very useful.
Readers, do you keep some emergency supplies in your car?