I'm going to second something Tweedle Adie said, about bringing books/stories on tape or CD.
I'm enough of a couch potato to think that having a portable DVD player, or a laptop that plays DVDs, is also a good idea. But I'd hate to think that all that scenery would never get noticed by a child because they're too busy watching TV to notice the world outside the window. And there should be at least a few times during the car trips where there could be something you can all enjoy as a family. And stories on CD you can all enjoy together is a great thing to share. You can also have some Disney music CDs handy, too or CDs of clean, Bill Cosby-esque comedy.
I don't have a kid, but my fiancee & I drove down and back to WDW from central NJ twice last year, plus drove back after flying down in the summer of 2004 to avoid a hurricane. We had Sirius satellite radio, and spent a lot of time listening to their Old Time Radio station. Thrillers like Suspense and The Whistler, crime dramas like The Shadow & Boston Blackie, and comedies like Jack Benny & Burns and Allen were a lot of fun for us. While they may be a little scary, there are scary in a PG-sense, not terribly graphic, and allows all of you to use your imagination (far more than that awful Drew Carey "Sound Dangerous" deal at MGM). They also have Radio Disney & kids' music stations. There's a "Clean" comedy network, as well as their uncensored "Raw Dog" comedy station. And of course, if you want her to know how horrible the world can be when she gets older, there's always Howard Stern.
You can pick a short-story or old time radio show to enjoy before or after bathroom or meal breaks, it gives you a story to concentrate on (so you're not getting on each other's nerves) and it also gives you something to possibly talk about later.
Many "plug and play" satellite radio receivers use an FM frequency you can hear in your car radio, plus a headphone jack, so she can hear something indepedent of what you're listening to if you want to listen to regular radio or a CD.
And of course, there's nothing wrong with bringing along a book or two. When I was a kid I could be counted on to have a bag filled with books for long car trips, and more often than not, I'd finish all of them. Granted, we didn't have DVD players or laptops then.
Also, as far as movies go...I don't know how much you'd want to spend on keeping your child entertained on this car ride, but if you're thinking about buying her an MP3 player, anyway, itunes now offers downloadable Disney movies, so you wouldn't have to worry about packing all those DVDs (just worry about an 8-year old losing a 400 dollar mp3 player with a hundred dollars of movies on it, haha). There's also an ipod-esque mp3 player out there I treally enjoy, made by Creative Labs. It's called the Zen Vision: M. On top of all the similar features ipod has, it also has a built in FM radio, a voice recorder (for fun singalong or Q&A sessions in the car), And, if you have Tivo, you can transfer files from your Tivo to your computer, and then convert them into WMV files that the Vision: M can play. I often convert some of my favorite TV shows and watch them on the train ride to and from work. Plus, you can always a get a cassette adapter or FM modulator to hear the audio from the MP3 player in the car's radio.
If after all those options, your child is still bored, just leave her at a Cracker Barrel.
