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.
The shawl looks lovely. And, it’s good to know how productive you are during traveling (I’m a fan of flight naps).