Maps, Maps, Maps
You'll probably start your trip with a planned route, but be sure to take maps of the areas you'll be driving through, just in case you need to take a detour to avoid traffic or simply go exploring. An atlas or a traditional map will do, or you can print custom maps for your journey at Web sites like GoogleMaps or Mapquest Route Planner.
A Global Positioning System (GPS) unit is a handy gadget to take along, as well -- some travelers say they wouldn't leave home without one! Many cars have navigation systems built-in, but if your car isn't factory-equipped, you can pick up one for around $100.