Fisher Price's Travel Doodle Pro stays in our car at all times. After learning the hard way that a 2 year old should not have crayons while in the vehicle, this doodler has been the perfect solution. A year and half later, my little guy still plays with it.
Another option we love is the Aquadoodle Travel N Doodle. Great in the car as well, though you have to remember to have water handy to fill the pen. If you take this one on a plane, remember not to fill the pen until after you pass security.
A safely distracted child = a happy parent!