Cinnamon for smaller leaks in the cars cooling system.

I`ll second the raw potato for pierced radiators, one of the girls in the local LR club drove 150 miles with one after a branch pierced her radiator on an outing. Some minor topping up of the coolant was neccesary on the trip home, but a good field grade fix anyhow...

Some lengths of electrical wire for juryrigging lights and electrical fans directly from the battery may come in handy as well, atleast if you have a basic understanding of car electrics or drive a Rover 4X4...