We are a road tripping family.

We consider the 12 hour trip from VA to Disney a medium-to long one day drive. This summer our family vacation was an 18 day, 7k+ mile coast to coast rambling trip. That's actually more common for us than Disney.
My DDs are now 8 and 5, but have been traveling "big" trips their whole life.
We do allow a movie every day (and on long days one again in the afternoon). Each kid has their own loaded MP3 player (music ONLY; not a ipod touch sort of deal). They can listen to that during any down time.
We make our kids trip journals - pictures to color, a page for each state, a map, activities. On longer trips we break them out over time. I don't know that your older kids would want all of that, but
On big trips we stop at every "Welcome to {new state}!" the first time we enter the state. It's cheesy, but it's something for the kids to look forward to.
We look at RoadsideAmerica.com and other similar resources to find off the wall stops or sights to be on the lookout for. My kids and husband love all of the kitschy stuff. I'm not as thrilled, but love how excited they get about houses covered in cans, or a peach shaped watertower that looks like a butt
We get new books, new stickers, new magazines, new toys - kind of. We set aside books, little toys, magazines well ahead of time, so that on the trip they are new again. This works for younger kids; not sure about your kids' age!
The kids look out the window a lot. We look at guides for places we're going. We buy postcards, write them on the next leg of the trip, and drop them in the mail at the next stop.
When a kid finds something that "works" for them on a trip, we stick with it. In a coloring mood this year? We'll stop and get more coloring books. Can't stop reading but are now out of books? Let's stop by this used bookstore. Learned how to make friendship bracelets? There's a Michaels up ahead; let's get you some more materials.
We play a lot of games - Who Am I? 20 questions. A-Z. I'm going on a picnic.
Ultimately, though, road trip in a car is just family time... in a car. We do pretty much the same stuff we do at home. Limited screen time, encourage interests, don't tolerate whining, be on the look out for quirky things to try... same at home and in the car
Have fun on your trip. Look forward to the ride. Kids take their cues from you. Let us know if you find anything awesome roadside - you know we'll be taking another road trip soon, and are always looking for new quirk
