I have logged a lot of hours on the road with my family (masking tape down the center of the back seat). This is the summer of college tours so we will be doing even more. I choose to drive because then I don't have to be the one turning around to constantly monitor the food and fun in the backseat. Now that the girls have their music for the road, things have gotten a lot quieter back there, but there are some family traditions that they still enjoy participating in. I do believe that my family's love of brain teasers has been instrumental in their success on the SHSAT (it isn't why we did it, but they have years of experience with logic puzzles). The best test prep is an active mind.
50 miles. Every 50 miles there is a little treat, something to eat, a little game or activity, another chapter in Harry Potter on CD, a brain teaser or minute mystery from one of the many stoop sale books that we have collected. We add 10 miles for every time they ask if it is 50 yet, and we take away 5 if they have used the roadside clues (signs, mile markers or mph) to estimate how far we have gone (no fair looking at the odometer!)
"which one is not like the others" This is a game we have been playing since they were very small. The only age requirement is that the child understands rules and that this is a game for fun and not for winning. The leader thinks of four things - one of which doesn't belong and everyone guesses which one. Sometimes people guess the right thing but for the wrong reason. The answer has to be guessable by the people who are playing and even though there may be many right answers the only one that counts is the answer that the leader is thinking of. The more creative the quiz and the answer the better. People are thinking up new quizzes as fast as they are guessing.
ex. Gettysburg, Antietam, Yorktown, Manassas: answer Yorktown
guesses could be Yorktown because it was a revolutionary battle or Yorktown because it only has two syllables
ex. lettuce, potato, corn, apple: answer apple
it could be that the apple is the only fruit, or that it is the only one that isn't a body part (head of lettuce, eye of a potato, ear of corn)
Greatschools.org has some good articles about traveling with kids. Check this one out about games for the road.