Read a book, play a gamegear, do some work (Assuming you're a student or something), knock one out if you're in first class and you have those sides which can be extended to make a mini-cabin. Hit on a stewardess, ask if you can wear the captains hat, drink a bunch of champaigne and get wrecked. Sleep?
Buy a miniature sketch pad before your flight. over the course of the flight draw a game of Tetris frame by frame until the book is filled. After the flight you will no doubt be filled with a sense of accomplishment and pride... Oh, and you'll need to be stoned for this to work.
If you stay up For a very long time before your flight you can probably get a good enough nights sleep on the plane that's what i did at least on my trip from Boston to Melbourne 30 hours of flight time D: I slept through at least half of it
Some (or most) airlines have these little TV-screens in the back of their seats.
I recently flew from Amsterdam to Kuala Lumpur with KLM and they had very recent movies (I watched Kick-Ass and How To Train Your Dragon), TV-series (Like the Big Bang Theory), recent music, basic video-games (casual stuff though) and even software to learn a language.
I learned some Icelandic, why not?
I had a very good book with me, but to my surprise I didn't even touch it.
Nintendo DS + new game (I recommend Professor Layton)
A good book
See how many objects you can put in the hair of the person next to you while they're sleeping
Refer to the stewardesses as "Air maids"
Watch every movie available on those little screen things
Stare at the virtual map of the plane's flight for several hours
Wear your barf bag as a hat
Find the answer to the question "How much alcohol can you buy on a plane?"
Hum an annoying/catchy song loudly until it's stuck in everyone's head.