Leave while they are asleep (or at a time when they will fall asleep).
Bring snacks, stop along the way to run around and tire them out....then they can burn off some energy and not bug you as much the rest of the time.
Have a few small, inexpensive toys to amuse them, but wrap them individually and hand them out at intervals during the trip (every hour, or every two hours)
Books for them to read.
GameBoy.
Or the easiest way out (and my favorite for long trips)...get a portable DVD player. Ask around and borrow one from someone, lots of people have them now, so it should be easy to find someone to borrow one from. Bring a favorite movie or two, and a new one. If you can't find a DVD player, remember that most laptops will also play dvds, but might be a bit of a pain to hold on the lap.