I love to travel. I even love to travel with my four kids. I will not say that every moment of driving/flying time is filled with fun, educational, time-consuming activities. I will not say that we never use our DVD player to calm the ants in the pants. I will not say that the kids sit quietly in their seats, content to look out at passing scenery. But I will say that the happy moments outweigh the grumpy ones.
Having a supply of games makes every trip better. My kids enjoy Car Bingo, especially the eeboo version that I save for "any trip that is longer than 3 hours one-way". Or, here's a printable travel bingo, and a bingo-like picture list that I like.
Alphabet game: Easily played by any kid who recognizes letters. When my kids are very young, I challenge them to just find the letters in their name, since these are usually the first ones they know. A variation we play when there are no road signs for miles and miles and miles (try Wyoming, or southern Utah) goes like this: name one animal for each letter of the alphabet--supersmartypants 10-year old has to think of animals with more than two syllables. Or, name places for each letter--anything from a town to a country to a planet is acceptable in our family.
License Plate game: This one is great for the cross-country trips (yes, I've done that; it was far better than I thought it would be). Just write down the name of the state as you see its plate; avid players may want to keep tally of how many from each state they see.
Favorites: Our family's all-purpose game. Take turns asking questions that everyone answers: favorite color, animal, movie, book, breakfast food, summer activity, school subject, etc.