I'll give you a hint. It's knitting.
Although my carry on bag also includes whatever work needs to be accomplished on the flight, my knitting is always in there as well. It's a real lifesaver for long flight delays or unscheduled overnight stops! And since I'm a somewhat nervous flyer, it's also great for distraction during bumpy landings.
The current carry on knitting is my Lida lace shawl.