You may want to take note of and try to recall activities that make your kids carsick (reading does for me), and don't emphasize those items or activities.
Gummy snacks are bad for the teeth. I am told that chewing gum isn't as bad.
Although a DVD player is nice, it is one more expensive item that could get broken in a moving vehicle, or stolen.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm