Oh boy. Haha good question. I fly a lot, these are my essentials:
-iPod. I don't know if you have one or not, but I have a video one so I load up on movies or TV shows to keep me interested. Or even just the music helps.
-Books/Magazines. I usually bring a book or two and I'll buy some magazines in the airport. The magazines are especially useful for when you're waiting for a flight.
-Crossword Puzzle Books. I use these types of books on road trips a lot, but crosswords and puzzle books are a great way to kill time.
-Notebook. I write a lot, but even if you don't write you can pass time with a notebook. You can do things like make random lists, or play games, or doodle or whatnot.
-Deck of Cards. Fun if you're traveling with someone. I went on a three week school trip to Mexico and we would always carry cards for the plane or the bus rides. You can even play Solitaire with yourself.
Headphones, iPod, book, snacks (check to see what is allowed to be brought through security), money for vending machine items and food once through security.