When you're travelling, the best games to take along with you are ones that you don't have to take anything for! The following three great games only involve your eyes, your brain and your surroundings. One assumes that you'll take your eyes and your brain with you anyway, and your surroundings are automatically provided...
I-SpyOne of the easiest games for travelling kids is I-Spy. The usual form of this game involves the words "I spy, with my little eye, something beginning with [letter of the alphabet]". This is fine, as long as your kids are old enough to know their alphabet! So when our kids were little, we used to go with colours - instead of "something beginning with [letter]" we'd say "something that is [name of colour]". This way, as long as your child knows their colours, or even just some of them, this game can be played by younger and older children alike.
When I was little my family went on lots of long road trips for our holidays. We used to look for windmills - not the Dutch kind - the Southern Cross kind. We'd count them all day long. Depending on where you are, you could count cows, red barns, houses (if you're a long way from anywhere!), billboards etc.
Southern Cross windmill.
Image courtesy of Matt Bridger, http://gallery.hd.org.
My children like to count yellow cars. They've been doing this for about 18 months now, and in all that time, they've counted well over a thousand. Every time they see one, one or both of them yells "Ack!" (it has become lost in the mists of time why they yell this, but they do...) and then the older one adds it to the tally.
Licence plate games
You can also play number plate (licence plate) games. Depending on the combination of letters you can see on passing cars or on the car in front, you have to come up with a phrase using the letters of the plate, as the initial letters of each word in the phrase. e.g. IWP could be "I wear pyjamas", "I want pesos" or "it will pour". You could try to come up with the funniest or most unusual phrase.
Another option is to use the letters of the number plate in a single word, in the order in which they appear. For example: "EJY" could be "enjoying". "TRV" could be "travel".
All of these games can be used in the car, waiting at airports, waiting in queues, and when watching the world go by. Don't fall for the mistake of getting just the kids to play them. Grown-ups can join in too!