I've done a bit of road traveling with my kids, now aged 2, 4, and 6. We live in a small town (technically a village), so when we go to "town" it means that we're going to be on the road for at least 40 minutes, each way. I've seen adults get more fidgety than my kids for this amount of time in a car; they're so used to riding for that long.
When my oldest was a baby, he couldn't stand being in the car. He'd scream the whole time, even if it was just for a mile to and from our house. So I started (for my own sanity's sake) to build up his tolerance by taking him for a ride every day. I'd go to the post office and then for a loop into the country and back, playing the same soothing songs on the stereo each time. It took about a week before he settled into the routine. And longer trips became a breeze, soon after.
We do have a DVD player in the car; like Siddow, ours is an aftermarket that we bought for less than $250. Surprisingly, the kids don't always watch it. It's been invaluable for the really long trips. My SIL has one that also runs on batteries, and until her son was old enough to sit still for longer periods, she'd take it even into restaurants.
I do the "trip bags" for the kids, too. I try to save things for different parts of the trip. And some treats are reserved only for road trips. My kids know we're having an adventure when we pack Little Debbie's pumpkin cookies. (Available only in the fall, but we always take our vacations, then.)
We love rest stops--they give the kids a chance to run, let them have a chance to pick out a snack (sometimes), and let them learn about the location we're in at the moment. We long ago learned never to eat at the fast food places with play areas. The kids don't want to eat, and the food isn't that great or good for you. They never want to leave the play area and they get really wound up while there, and they are generally unhappy when you tell them it's time to go--regardless of how long you've already been there. Stopping at those places will eat a big hole in your travel time budget, and you'll end up getting to your destination much later and everyone will be crankier for it.
We do take brief detours for local attractions, sometimes. We all get to learn something new and have a surprise experience. And it usually doesn't cost us much more in time or money.
We videotape parts of our travel, as well. If we hadn't, we'd never have gotten a shot of our son singing "Old McDonald" to himself. We'd also have missed capturing the general silliness and giddiness that comes of being trapped in a small space with each other for hours at a time. (Thankfully, no one has recorded the meltdowns that come with it. And the kids aren't always the ones who have the meltdown!)