As a parent, I have found that the worst part of travelling has been bored children.
They have an in-car DVD player (a gift from their grandparents), and their hand-held game-consoles, but every so often they decide they want to do something else.
Here are a few of the things we do.
Step 1: I-Spy
A long-term favourite that is even older than I am.
The driver starts, calling out the ritual chant;
With my little eye,
Something beginning with...
The rest of the passengers then keep guessing things until somebody guesses correctly, or they all give up. If somebody guesses correctly, it is their turn. If everybody gives up, the driver (or whoever said the rhyme) takes another turn.
Points may be awarded, or small sweets (this is a great way to make a bag of M&Ms last a lot longer than usual).
Allowances must be made, of course, for the age of the contestants. You could say Something beginning with a "wuh" sound, or you could say Something green. At the other end of the scale, you say Something cuboid" or "Something with four stomachs."
If you have small passengers, try and avoid objects that will disappear from view quickly. If you have quick-witted passengers, then you can concentrate on those same objects.