We have taken regular 12 hr car rides with my son since he was 3 months old to see my parents. Its never all that much fun, but once he started walking we came up with a pretty good system.
We maped out our stops before hand and made them either at rest stops where he can run around or at fast food places with ball pits and slides. We stoped about every 150 miles. In between the stops we fed him snacks, I climbed back there to read him books, play peek a boo etc. The portable dvd player was the best by far because it would keep him entertained for a while. We buy 2 new dvds each time of his then favorite shows, first baby einstein, then diego, blues clues etc.
Im not saying it was tearless, but it was better than without it for sure. The dvds are the way to go!

Oh don't forget books on Cd/tape also!

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A woman is like a teabag. You never know how strong she is until she gets into hot water.-Eleanor Roosevelt