Everyone has already covered the essentials.
But here's my list:
Pringles
Beef Jerky
Goldfish
Chex Mix
Werther's Originals, either regular or coffee
Some sort of cheese and crackers pack
Possibly a sammich
A cooler filled with ice (not ice packs, this is key)
Water
Arnold Palmers
A couple red bulls in case you get tired
And don't forget an ipod loaded with music and possibly a book on tape >_>
If i had all that shit on the 18 hour car trip i took last month, it would have been so much more bearable.