One suggestion is to head to your local dollar store and buy 20 different toys. Pull out a new toy when you first start the trip. After the kids get bored with the first toy, pull out the next one. By the time they've gone through all the toys, a good amount of time will have passed and you'll be that much closer to your destination.
Another great tip is to purchase some games specifically for travel. Amazon.com has some great travel games to keep the kids occupied from creative play toys like the Crayola Color Wonder Travel Tote or the AquaDoodle Travel Doodle to memory games like Melissa & Doug Flip to Win Memory Game.