With a long plane journey and an active two year old to keep occupied, I decided to buy the Crayola Glow Station.
It's a light weight board that comes with a light pen. It's small enough to fit into a bag and carry with you and large enough for plenty of creative drawing.
Switch on the light pen and your kids can draw glowing light pictures. The pictures fade after a few minutes so you can start again. It works in the daylight but is most fun in a dark room or onboard a plane.