We'll be driving to Disney too. I have a three year old and I try to bring lots of little goodies to make the trip more enjoyable for all of us!
As for making your daughter a map...perhaps you can get a map placemat from Wal-Mart. Then you can use a washable marker to draw an approximate line to outline your course. As you pass through each state, she can place a sticker on the state. We sometimes get free stickers in the mail (sheets of happy faces) and I save those for things like this. It would be a way for her to keep track of where she is...if course, you'd have to help her identify the states and tell her when you've entered a new one, but she'd probably still enjoy it.
Last year we brought: books on tape (my parents recorded a couple books for my son to listen to..he loved hearing their voices), stickers, paper, Crayolas mess free paints and markers(the fingerpaints/markers use special paper...the paint/ink only shows up on the paper, nowhere else), magnetic sets, a slinky (he LOVED that...crazy kid!), snacks and juice, a small pillow, couple of books, Hot Wheels car, and a Magna Doodle. I know I'm forgetting some of the things! I bought a car organizer that attached to the back of the driver's seat. It held everything and kept it organzied. We played games (he'd ask us to draw something on the Magna Doodle or we'd draw something and he'd identify it), napped, listened to music, looked out the window and let him play whatever he was interested in. We took a few breaks and arrived without crying or tears...so all in all it was an easy 10 hour ride.