Luna bars (or any protein or healthy bar), dried fruit, Japanese rice crackers, sesame snaps, 9-bars, pumpkin seeds...anything that you can put in little packets and not have to worry about smashing in your carry-on. Don't take too much, though. You probably won't get as nibbly as you think you will. (I always used to carry too much junky snack food. Try making a plan, like if your flight is 9 hours with a 2 hour layover or whatever, plan how many times you'll snack and when, and just take enough for that. Keeps you from eating too much, too).