My theory has always been that you always need what you forgot to bring, so make sure that you bring all the necessary tools for everything you absolutely don't want to happen.
- If you don't want to run out of fuel, carry a fuel can with extra fuel.
- If you don't want run into a snow, carry chains.
- If you don't want an engine belt to break, carry a spare.
- If you don't want fuses to blow, carry extra fuses.
- If you don't want a light to go out, carry extras. (I'm sure you get the idea by now)