We drove from PA to FL, and then back again. And from PA to SC, and back again, and not a single "Are we there yet!".
The first trip, they were 2months, 6 and 8, and the second they were 1, 7 and 9.
What we did was have the girls each pack a bag, and bring their DSs (though on the FL trip.. they forgot GAMES for them, and just packed the DS and plugs

). We also made them little presents and stuffed them into paper bags, one for every state line (or 3 hours, whichever came first... because VA especially is a longggg state to go through!).
So on our way to Disney, their bags had stuff like new crayons and coloring books in one package, new toothbrushes and toothpaste (Disney themed, of course!) in one, Sunglasses in one, sticker and activity books in one, etc.
Trying to think what else we did.. oh. Bows and necklaces were a big one. Personalized water bottles. A few new books. We got them each their own hairbrush, which was a big thing for them, because we always share one
But anyway, they were always so excited to mark states off and play with their new stuff that they didn't complain!
And we generally put a small snack in the bags too.. fruit snacks, dried bananas, fruit pouches, etc. It worked out so great!
We also stopped and made everyone do stuff like 10 jumping jacks, running in place, lunges across the grass, etc. That helped everyone with the antsy "stuck in the car with your knees in your mouth" feeling.
