I know it means a bit more work for you, but we always bring not only books for our children to read, but new books for us to read to our children -- maybe a chapter book in a new or continuing series?
Food is always entertaining. We like things that take a while to eat, like grapes, pretzels, cereal. We haven't had trouble with me packing these in my purse and carrying them on the plane.
Ditto the DVD player and DS. Both totally worth the money if you don't already have them.
Other ideas: maze books, an ipod with books on tape, sticky notes and/or scotch tape for endless fun, Crayola Model Magic, playing cards, Highlights Hidden Picture Books, and action figures. HTH!!
