I always make a travel bag for my grandson, coloring book, crayons, stickers and sticker book, magna doddle, small toys whatever he is into at the moment... like dino's, cars, action figures, Hit the dollar tree, target dollar zone, and wal-mart of course
Since they are driving... when our DD's were young and we would drive long distance, I always packed a ball or Frisbee in the bag and at rest areas let them burn off some steam, so that they did not start getting on each other nerves or ours...
Another thought for each day is to give them some small new toy or something for them to do in line, some thing new will keep their attention, then at the end of the day, put it away... then something new the next day... You can make your own play packs...
One more thought... I would load up on i-pad, Kindle kids, some movies and such for the car ride, if you can let them each have their own this way no fighting on who is holding or not agreeing on the movie or game...