For me it depends on the length of the flight. On a short one I usually get the USAToday free at my parking lot or the US Airways Club and skim that then do the crossword puzzle. On a longer one I will pay for the New York Times and read every article then attempt to do their crossword puzzle. The Times usually takes me about two hours to fully enjoy so that helps. Following that I'm a big reader and as I'm now pursuing a Master's I've been bringing my GMAT prep book with me. As a noted procrastinator, being on a plane allows me to get more of that done then when I'm home.

Alternately, if I'm flying American I use the seat power outlet to connect my laptop to power and then I will usually watch a Netflix movie or edit photos.