Travel toys

Travel legos (we actually use a set of megablocks that come in a nice box that can be used for buiding; these have the advantage of being cheaper so we aren't as concerned if we lose any)
Reuseable stickers (like Uniset)
Travel spirograph
Travel magnadoodle (these have to be treated with care lest the screens be damaged or, as happened to us, the glue holding the plastic together gives way)
Travel desks or clipboards
Coloured pencils (we prefer these to crayons, which can melt in the car, or markers, which can either be ruined if the caps are dropped or ruin clothes if they drop on them and soak in)
Walkman and tapes
Travel Boggle Jr. (a bit big for a plane trip but fine for the car)
Wiki sticks
Little presents (stickers)
Books

Polly Pockets (miniature worlds with tiny people; for kids who don't put things in their mouths and are old enough to keep the people from being lost)
BrainQuest (like flashcards)
Coloured pencils and paper
Finger puppets (we got ours at IKEA)
Books