What can you do on an airplane to pass the time? Beyond napping, my favorite distraction is to look out the window for an atmospheric phenomenon that is visible if you look in the right place.
It starts with choosing the right seat. When making your seat selection, pick a window seat on the side that will be opposite the sun.
As the ascending plane breaks through the clouds or as it descends toward the upper cloud deck, look 180 degrees from the sun to see if you can find a circular rainbow. If you're lucky you may also see the plane's shadow within that colorful halo. Woohoo!
Thanks to Jessica Baron for reminding me of this notion.
[Added Feb. 29, 2016: Aaron McNeely noted that this phenomenon is called a glory. Indeed it is, and rightly so.]