Video Games
If your child has a personal video game device, this can be a great option for them on a flight. Research game sites online to find appropriate games for your child's age range and make sure the device is charged before you get on the flight.
Activity Books
Coloring books are a good activity for younger children. Crosswords, puzzle books, brainteaser books and Sudoku make good games for older kids. Mad Libs are fun to play as a family activity.
Family Games
Some families like to play games together on the flight. One option is to play "I Spy" or organize a mini scavenger hunt, where the kids have to find various objects around the plane. You can also create an airplane bingo sheet before the flight, where each square is a different thing that might happen during the trip.
Mini Board Games
Many companies make airplane-friendly versions of board games. These travel versions are easy to pack and make good diversions for all ages.