I discovered this while camping, and then again while rushing out the door to get my late-risers to school: oatmeal is the ideal on-the-go breakfast, especially when served in an insulated travel mug. Oatmeal stays hot, can be eaten in the car without spilling, and doesn't leave crumbs everywhere.
Bonus: many insulated mugs are geared toward commuters…so fit nicely in the car's beverage holder.
If you use instant oatmeal, all the quicker. Instructables shows how to make homemade instant oatmeal packets.