I keep a Rubber-Maid container in the back of each vehicle with the following stuff in it:
enough oil for a complete oil change and spare filter
jumper cables
hose repair kit
2 cans fix-a-flat
flashlight
radiator stop leak
flares
spare fuses
small tool kit
folding shovel
can of brake fluid
1 gal of anti-freeze mixed to 50-50 with water
can of starting fluid