We recently went on a 10 hour car trip this summer with our 20 month old & 4 yr old. My mom came up with this idea and it worked great....aside from all the snacks, movies etc. she bought 8 little presents (for each child), wrapped in tissue paper, for the kids and we passed them out throughout the ride. Nothing was expensive (crayons & paper, little toy car, etc) but the excitement of unwrapping & playing with the new "thing" worked at keeping the kids happy and occupied.
Have safe & fun time! :)