Back when I flew more often (I've since quit altogether, because nothing I do needs teleportation and because I feel like I've used enough jet fuel thus far), including some round the world trips, I would sleep, read a book (this was before phones and ereaders), get up and walk around, look out the window, daydream, maybe watch a movie (or just start one, or effectively watch most of one playing in a seat ahead), maybe chat with a seatmate. I like the act of mass transit, in part because it's so final; if I forgot to turn in a term paper, oh well! Can't be helped, I'm on a plane now, just relax.