Things I keep:

Mace (for bears)
Radar Detector
Baseball bat
Mints
A Map
6" knife
Portable Fridge
Wet Ones
Umbrella
Portable Vacuum Cleaner
Pillows
A Blanket
Tools
Spare Cash stashed in weirdest parts of the car (don't ask)
Cellfone Charger
Cologne
Box of Tissue
Some tylenol, advil, claritin, immodium, gas-x, maalox
first aid kit
A Bottle of Redline Fuel Injector Cleaner
Glasswipes