* Sleep (ideally closer to the time zone of your destination) - often useful to have a neck pillow, eyemask, earplugs, and sleeping pills (which are all far cheaper in a local pharmacy than in the airport)

* Talk to your neighbors if they seem friendly (sometimes people really enjoy this and end up unexpectedly talking for several hours)

* Bring a variety of different media or tasks like different genres of books, different problems or puzzles, etc., perhaps movies or music, so you can switch around from one to another (... including creative tasks like writing a letter, an essay, or a computer program)

* Maybe try meditation, or thinking about particular problems or questions that you've thought were interesting or worthwhile for further reflection