I haven't been on a 15 hr. flight though, but I have been on JFK-TLV which I believe is a 12 hour flight(that is my longest).I don't really remember what I did on both legs other than sleep and watch ptv.
On ORD-ZRH, I was in business class, so I watched part of a movie, ate, and went to sleep.
LGW-DFW I was in business class again, I watched a little tv, ate, and then slept.
ORD-LHR, it was a 10:30 departure. I essentially went right to sleep after take off, and awoke about 1 hr. before landing.
LHR-ORD, it was a morning flight. It was hard for me to sleep, so I looked out the window at the ice bergs and greenland. I watched the inflight movies, listened to my cd's, read some magazines.
What I think is really cool is when flying east-bound, it is neat to look out and see either the constellations and stars, or look down and see the city lights. For some reason, I think it looks really neat. There is also the occasional ship lights in the water below you.
It is those times when I have really felt that I was crossing the globe.
Give me a break, I created this username when I was a kid...