One thing you have to count on is that a child that age will sleep. That's great while you are still moving, but remember that when you finally stop, all the grownups will be really tired, but the child will be bursting with energy and raring to go. If you are stopping overnight, I recommend choosing a place with an indoor pool; great for getting rid of those "cooped up in the car too long" fidgets. Another good thing to do is keep your eye out for playgrounds along the way, and stop for exercise breaks occasionally.
Our DS always liked flash cards for trips; he loved flipping through them and identifying the pictures. He also adored the Dorling-Kindersley picture board books that show lots of photos of various categories of things; he had one of animals that he never got tired of flipping through. He was never much for coloring, really, but he did (and does) like magnadoodles and colorforms, which can be played with on the windows. Also, those little plastic folders with re-usuable stickers that form scenes are good, too. If you can stand listening to kiddie tunes, I recommend the Disney CD's; a personal player might work, but not all kids can manage the headphones at that age. We play "name that movie" with the Disney Classic CD's; it's a good game for the road.
Oh, danger warning on the coloring; get fat colored pencils instead of crayons; crayons will melt on your upholstery. If you have a Marshall's store nearby, try there for kid goodies; they often seem to have a lot of good stuff at nicely discounted prices.
