My parents did these things when I was a child.  If I would have had children I would have also done them.  Very good suggestions tourist.  I can add a little from what my parents did:

1.  Coloring books and crayons, colored pencils, and/or markers can keep children occupied for long periods of time if you have children that will only draw or color on the intended coloring surface.  If you have "wall writers" you'll want a less messy toy.

2.  If you prepare your own snacks, you have more control over healthy snacks.  Fruit, juices, veggie sticks and cheese are all easy snacks and much better choices than the candy bars, chips, and sodas you'll end up getting at convenience stores along the way.

3.  My parents always got brochures for places we'd be passing along the way.  Things that would be both entertaining and eductional.  There was always a museum or historical site somewhere along the way.  Gettysburg stands out in my mind even now.

And one last thing I can add, portable dvd players have become more affordable lately.  You can get a car plug and bring along some favorite movies, or.... if you find dvd's available pertaining to your vacation (I know Disney has them, and I'm sure other places do also), they'll add to the excitement.

Fortunately I don't have this problem. However I always thought that the car TV's are a wonderful idea. I take a lot of road trips and I think it would be really hard to keep children occupied for such a long time.

Buy a car with Rear Seat Entertaiment to keep your kids busy

I have four children and long holiday trips where always a nightmare before we installed portable DVD Players. Before that it was always every five minutes when are we getting there? How long now? I'm bored? We always play eye spy now that keeps them occupied. And another good hint is to always insist on toilet stops when ever you can manage.Oh and the other good thing that has saved us is the invention of the Ipod and MP3 players excellent for long trips. I love them.

What about a PSP (play station portable). I think you can keep your children occupied even longer than with a movie. If you have two, they can play against each other, but make sure they won't argue, otherwise you have achieved the exact opposite. 

Andre

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I think that almost anything that your children likes to bring can be brought in a trip.  Even if they are not in the plane, parents  need to bring these things still, e.g., PSP, coloring books, gameboys, and other portable games.  So,just think of things that will entertain them which are also portable, and that's it, problem solved! :)

ive just bought a twin screened dvd player for the car and it is a god send a must for any parent, keeps them quite for ages.