I agree with Limey about keeping the "stuff" to a minimum. Crayons & small colouring book is good. Those ones you get with a single texta that goes on invisible then reacts with the paper to make colour are good, small too. But the thing I was going to suggest was a miniature magnadoodle. They are travel sized, usually have a clip, the pen thing is attached, and it can be drawn & erased over and over and over again. Handy afterwards in the car too.