Ooooh! I can answer this! We drove in Dec from MN to WDW. Two long days of driving and it went really well. I have a 6 yo and 2.5 yo as well as a 1 yo.
We made sure we had several new CDs of kids music. Also lots of story tapes from the library. Each child had their own backpack with books, color wonder markers and paper and a few snacks. I had LOTS of little presents ready to go. These included frontpacks with pez dispensers in them, magnetic books, a story tape for each, a new book for each and gosh I don't remember what else. We actually only went through about half of what I had on hand.
We sang songs, we played Simon says, we made sure that everyone had snacks and drinks. A special treat for my kids was that on two of the days--one down and one back--they got to go into the gas station and pick out a snack. We made sure on the way down that we stopped every couple of hours so everyone could get out and run around, but the weather on the way home was rainy and snowy so we just drove and drove. That actually worked out better.
We have a minivan, so we were also able to do a little seat switching as the 6 year old is just in a booster. that seemed to break up the monotony a bit getting to sit by a different brother or taking turns sitting by mommy.
My kids also have Leap Pads--the regular one and the My First--and the 6 yo also had a GameBoy although she actually didn't use it that much. She had a walkman to listen to longer books on by herself. (The two year old isn't into the American Girls series)
I think that's about it. Let me know if you have any questions and I'll tell you what I can.