Forget a favorite blankie and you'll wonder what you did to deserve hours in the car with a screaming child and no respite. Bring the right stuff with you in the car and the kids will enjoy the trip as much as any destination.
Give each child a small backpack and let him or her choose what to put in it. Your child may want bring a favorite stuffed animal or blanket or book or toy. Just be careful that you don't bring anything that would be devastating to lose.
Choose toys carefully. Good car toys include magnetic games, dolls, action figures and books. Toys that are better to leave at home include anything pointy or with sharp edges, toys with small pieces that can get lost and toys that make loud noises.
Music can soothe the savage beast. Bring personal stereos with headphones for each of your kids. One can listen to N'Sync and another to Pink... all while you're listening to NPR or Billy Joel. And don't forget lots of extra batteries!
Help your kids record their memories of the trip. Buy each child a journal and remind them to write a little something each day. Buy post cards in the different places you visit; the kids can tape them into their books. Give them each a disposable camera to take the pictures they want to take... just like Mom and Dad.