One thing that several parents (including my sister) have found to be a great diversion on a plane is a bag of colored pipe cleaners. They are quiet, have no smell, make no mess, and can occupy a child for hours--especially if they are something the kid has never had to play with before. Get some of those sticky backed "eyes" from the craft shop as well.
Another idea I saw here on the DIS once was various shapes of colored felt and a piece of sand paper--they work like Colorforms but are less expensive.
Anne