While it is best not to plan too much since this maximizes fun and chances to learn from your mistakes, it is also a good idea to bring some necessities. What you should bring depends greatly on where you might be going, what season it is, and how long you are leaving. As a rule of thumb, I try to pack the following items:

Garmin Etrex Legend This cheap GPS unit is very useful since it displays a general outline of the local main arteries. It also acts as a compass, displays your average speed, and leaves a trail of bread crumbs for when you get lost in the forest like Hansel and Gretel. I did not know how useful it was until I was lost in the Pisgah National Forest and it showed me the way out. You NEED a compass because parts of this country look the same no matter which direction you are heading and you cannot count on the sun or the stars if it's cloudy or while under a thick canopy. Another good use for the Etrex is to show you your progress on the road. I prop it up, wedging it between the windshield and the dash, and keep an eye on when the next town is coming up. It also lists services like gas and food at each Interstate exit. I never leave into the unknown without it anymore.
Rand McNally Road Atlas At stores like Wal-mart or at some truck stops you can pick this up for about $5 and it is the best $5 you can spend. I prefer the large type one and one with binder rings is always better. I abuse my atlas so much that it never really lasts through more than two trips. These atlases have the clearest and most accurate depictions of major roads in an area. Rand McNally maps are the best for traveling along the Interstate and list every exit accurately. City maps, mileage guides, and other information in the book is also very useful. They used to even have a toll road guide that told you how much each road costs to travel. While driving, I keep mine open to the correct page between me and the car door for quick access.
DeLorme State Atlases These atlases are a bit more expensive and should not be man-handled like the Rand McNally. These display every town (former and present), every back road, exact locations of campgrounds and geographic features, show topography (important for planning your travel time), and even list local landmarks like towers and electric lines. With a GPS and a DeLorme atlas, you will never be hopelessly lost. The maps are uniform and make local planning a breeze. It is not good to read this kind of map while driving and it is not useful for Interstate travel since you will have to turn pages too often, but is great for those blue highways. It is always wise to know when your concrete road might become a strip of gravel.
Pouch for local information Do not just throw it up on the sun visor like me. Especially for long trips, stay organized. You will collect a lot of free local maps at rest stops and welcome centers that are essential to local travel, especially through towns and cities. You need a good place to store any directions and notes you made before the trip began. Keep your information in ONE place so you will know where it is when you need it. When you have passed an area, do not be afraid to throw stuff out that you don't need anymore. The mess can build quickly.