Reading is a common method of entertainment on a long plane flight. You may find it useful to bring magazines or short story collections, as a long plane flight can adversely affect your attention span. If you do bring a full length book, you may want to stay away from complex tomes, and stick with "airplane reading" such as simple fiction or mysteries. Make sure to bring backup reading, in case you get restless or bored with the first book.
Another method of whiling away the time on a long plane flight is the use of puzzles or games. Books of word games are readily available at most bookstores, especially in airports. Physical puzzles, such as a Rubik's Cube, may also be something to consider. Doing something with your hands such as knitting, drawing, or working out a puzzle can be an excellent way of distracting your mind from the long plane flight. Be aware that many airlines restrict the items which you can carry on board, so consider whether or not a hand diversion can be used as a weapon before packing it in your carry-on.
A long plane flight is also very physically demanding. In addition to getting hungry, your body also undergoes physiological changes which can be harmful if they are not addressed. Make sure to pack snacks to keep your hunger sated, and drink a lot of water to avoid dehydration. If you get nauseous on planes, stick to bland foods and carbonated drinks to settle your stomach.
It is also very important to stretch and move around on a long plane flight, to avoid cramps and a condition called deep vein thrombosis, a blood clot which can form when you remain stationary too long. If unidentified, the clot could travel to your heart, potentially killing you. Every hour or so, get out of your seat to move around, bend your limbs, and stretch.