I made a 15 hour trip last month by myself with my 22 mo and my 4 yo. I expected a nightmare, but the trip wasn't as horrible as I anticipated. Here are my must haves:
- snacks. We had an only water rule for the car and I tried to buy snacks that wouldn't be too messy. Eating kids were happy kids.
-DVD player. Helped a lot, but my kids didn't watch as much as I thought or hoped they would
-ball. We kicked it around at rest stops to blow off some energy
-surprises. A mom on this site suggested having random surprises on hand. I stocked up at the Dollar store on treats. When the kids got fussy or bored I pulled out the new toy. They may have only played with it for 10 minutes, but it was a big enough distraction that they stopped whining.
-pillow and small blanket
-playgrounds- we ate at places with playgrounds
-Toys. Stored them in a basket. Etch a sketch, magnadoodle, books, crayons, coloring book, tag reader, etc
-Lap tray. http://www.target.com/Eddie-Bauer-Travel-Tray/dp/B0033A42JE This thing was a life saver! I originally bought it for my baby to play on. Since it's not hard enough to color on, I didn't get one for my son. We wound up stopping to get him one. He could play with his figures and cars without them falling all over the place. He loved it and I didn't hear anymore about dropped toys. ( A friend borrowed mine for her daughter...she played with her LIttlest Pet Shop figures on a long trip.).
HTH! Have a safe trip!