Sing (or Read) Along
There's something better, much better, than the sound of silence. It's the sound of a child singing, reading or laughing, and all of those things can pass for "quiet" when travelling. Who's going to object to a soft rendition of "Twinkle Twinkle"?
Whisper singing is a great way to pass the time on a plane without disturbing the people around you; in a car, volume is less of an issue. Parents can join in for a sing-along that can keep a child occupied (i.e., not yelling, crying or whining) for quite a while.
Reading a book together on a plane is another great quiet-time activity. In the car, reading can be problematic -- if you want to avoid the sound of vomiting you'd be better off listening to a kids' book on tape.
Up next: Novelty never fails…