I can't do anything productive or mentally taxing on long flights, I don't know if it's the air pressure or premature jetlag or what, but I have a very hard time exercising my will. What I do enjoy, though (in addition to the Marvel comics movies – I only watch them on planes) is reading documentation. It's a great time to sit down with an in-depth manual and just soak up information you didn't have before. You don't have to do anything, or make anything, you just learn how something works. Another related activity is reading a bunch of code that I know I'll be working on in the near future. There's something very pleasurable about absorbing a codebase while at the same time completely excusing myself from actually doing anything.