A happy child is an amused, well-fed child. For kids who are old enough to carry their own backpacks, include an assortment of toys, electronic devices, and snacks that will stand up to the long haul. Granola bars, nuts, fruit leather, and the occasional sweet (M&Ms travel well) won't get squashed or stale on an arid airplane. As for hand-held devices, skip the donut-making/Angry-Birds time sucks, and opt for Travel Bingo, National Geographic's National Park Maps, and PicPocket Books where you can download favorite books.
Successful travel toys are reusable and won't roll off tray tables: triangular crayons, mini Lego sets, washable markers and drawing pads, and digital cameras for the over fives. Travel activity books like Rand McNally's Are We There Yet? and Mad Libs on the Road will get them excited about explorations both large and small.