Hi A.,
I haven't read all responses, so forgive any duplication. Shannon S has many good suggestions--especially Talking!
Just watching out the window is something to do, because there will probably be some sort of change in the landscape if you are traveling that far. We have an awesomely beautiful country, he has a great opportunity to see some of it. Many kids will never have a chance to take a trip like that.
Does your child like drawing? or writing? bring plain paper and pen/pencils, and/or a journal. My youngest (now 14) still does origami on trips sometimes.
CDs to sing along with. Or browse the radio stations and listen for different accents of the announcers, different content of news reports (e.g., here near Lake Erie we get small craft warnings, in farm country you get corn prices).
I like the idea of coloring in the state on the map when you spot a license plate from that state. Or you could just keep a list.
When we went to California from Ohio when I was 12, I kept a list of all the rivers we crossed.
Show how to read a map, and when he asks "where are we?" tell him "we are on highway x and we just passed the junction of highway Y". He can make a mark on the map. Then in 15 min. when the question comes again, You can say "we just passed the junction of highway Z" and he can make another mark.
some word games that can be played by the driver as well as passengers--I forget the names, but one goes like this: First player starts out thinking of a word and says the first letter for example "B" (for black, but you don't tell the word). Next player adds a letter for example "A" (because he's thinking of "Bat" but he doesn't say that). Next player might add "C" thinking of "Back". Next player will probably say "K" for back--this player loses because he/she has finished the word. This player could also have said "T" thinking of bacteria. If challenged, the player must say the word s/he is thinking of If other people don't catch on to what it mught be. It is interesting to see how the words kind of "Twist and turn" as you adjust to the letters put forth by other players. I wouldn't emphsize the winning/losing aspect, more the fun of "Gee we sure thought of a lot of different words!" aspect.
Alphabet sights--first person looks out window for something that starts with A, next person looks for something with B, etc.
Alphabet lists--see if you can come up with a list of flowers starting with each letter of the alphabet, e.g. aster, bluebells, carnation daisy,.....zinnia.
Try alpha lists of other stuff--states, countries, sports, foods, board/card games. He can keep a list on paper if he wants.
Travel scrabble game--no, the driver can't play along, but your child can arrange the letter tiles to make crosswords. At your destination you and he can play it the regular way.
Is there a travel bingo game? You could say the numbers randomly. He could open a treat bag when he gets a bingo.
Look for pictures in the clouds.
Give him a disposable camera so he can take his own pictures. Remember to stop at the scenic overlooks (at least some of them) so he can take a photo (and stretch his legs).
Have him start a letter to a friend at home telling about the trip. He can finish at your destination.
Does he need to review his math facts? If you are the only other adult, just ask him orally (You can't exactly do flash cards while driving). See how many in a row he gets right, and congratulate him on that, raher than dwelling on the inevitable wrong answer. If there are two adults, one can drive and one can do the flash cards.
Does he do sudoku? print off a bunch of puzzles from WebSudoku before you leave.
Other word tricks--see how many words you can make from the letters is one word--e.g. "Charm" has charm, harm, arm, char; if you re-arrange letters you can get car, mar, Marc, ram. You can do this orally, or he can write on paper, dry-erase board, whatever.
Make up your own stories. One person starts, leaves off at a particularly exciting point, and the next person continues. You can even do sentences this way and see what kind of funny stuff you end up with.
Tell jokes.
Mad-libs these can get hilarious!
If you see an unusual house, or one in an unusual spot (e.g. the top of a hill) speculate about what it would be like to live there.
Use your imagination, I am sure you will have fun. I am envious--I wish I were going too!
K. Z.