We just got back this week from a 20 hour one way car trip. You have my sympathies and I can totally relate.

A couple things we did - I had a cup available to put markers or crayons in (only allowed one out in the car at a time) and they fit right in the kid's cup holders in our van. If they can reach a cup holder, that might work for you! We had much better luck with not losing crayons or markers this trip.

My daughter likes magnet play sets (she's 4). We also got her a sketch art book that she loved. (there are lots of these available at the book store)

http://www.amazon.com/Princess-Bella-Scratch-Sketch-Princesses/dp/1593599722/ref=pd_bxgy_b_img_b

My son thought it was so cool, he found one for older kids at the zoo we went to. These are nice because all they need is the sylus that fits right into the book. The younger kid version was like tracing and scratching in the lines. The older kid version was free drawing. My 4 year old also had a couple sticker books that she liked a lot. I hid several things for her that didn't come out in the car until we were in need of something new.

In the car, I kept a refillable water bottle and a snack cup with a cover for each kid we'd just refill.

My 8 year old read a lot (he finished the Harry Potter series on our trip) and he kept a trip journal where he'd write everyday and he'd glue in our tickets, brochures, etc.

We've also listed to audio books in the car which has been really great. My husband has a mechanism to use his ipod with a tape player in a vehicle. We've had mixed results with the 4 year old on this. But my son would listen happily for a LONG time.

We also have a DVD player, a nintendo DS, and a leapster so we abused technology a little bit too. Good luck and have a great trip!