Portable DVD player - most important
I got some cool wet books from Kohls over the holidays but you can get them anywhere. They are hard cardboard, and they come with a little container that you put water in and it has a paintbrush tip. They paint the pages and the color shows up. THen it air dries and they do it again. The only down side is keeping water in the paintbrush, that takes a little skill. You have to do it of course.
Instead of crayons, I got mine FAT colored pencils. That way the tips didn't break and the colors didn't melt. I got them fun pads/ or just plain note pads for doodling.
a good supply of snacks - I've heard grapes ward off thirst, cheerios, animal cookies, things that are going to destroy your car.
Be very prepared to take a lot of breaks. Plan at least two good stops where the kids can get out and run around for a little bit. Like the state rest areas that have playgrounds, or look for a park on the route to where you are going. \
Plan your eating locations with your kids in mind, make sure it's a family friendly resteraunt, so they can get some play time there too.
Patience, if you can find it and pack some of that it would be great. Because the last 2 hours of the trip will be horrible.
Oh and children's books on tape. You can get something for the library.
We make a regular 10-12 car trip once or twice a year. I have a 6yr old, 3 yr old and 6 month old.