Wrap a bunch of little things (even snacks) for the backpack. As a reward for them, or a help to you, they can unwrap them periodically.
Sponge (nerf) ball, if you don’t mind them tossing it in the car.
Something to play with (like a ball) at the rest stop.
On long trips, books on tape/CD are my friend! You can check out a whole bunch from the library. Either ones you all listen to, or if you have individual players, ones they can listen to by themselves.