My dd9 loves to have an atlas so she can mark where we are. At rest stops she likes to pick up various attaction pamphlets to look at.

along with the ideas already given, my dds like wikki sticks, magnetic games, color wonder markers and paper (no mess). We also got the "wiggle seats" that they can sit on for a change in position!

They bring Ipods (we load audio books from the library.)