Rob,

A hammer. You must have a hammer. And a flashlight.

As the owner/driver of two older vehicles (86 Suburban and 82 VW Rabbit P/U), I keep tool kits and spare parts in both.

Also,

Wrenches---socket, combination, allen, etc.... Vehicles are held together by nuts and bolts. Having just a crescent wrench is asking for headaches. Excedrin headaches.

Hose clamps, wire ties, duct tape, wire nuts, hand cleaner, a rag or two, spare belts, hoses, fuses, a quart or two of oil. Water, snacks, warm clothes, blankets, etc....

Lots of other stuff.......those with well maintained newer cars have lots less to worry about than those of us with beaters.

Take care,

Andy