When the toys get old and they're not hungry, play lots of games. Work hard to keep your child focused on something besides getting into mischief. Maybe it's "I Spy," "Peek-a-Boo" or "20 Questions," or maybe it's talking about how an airplane works or what you can see from the window.
Eventually, your child is going to have to move around. Walking up and down the airplane isn't always possible, but kids just aren't cut out for sitting still for hours, so get up and move around with them when you can. Just make sure that they're respectful of the other passengers.