1. Word games. Have your kids keep their eyes on the license plates and billboards passing by as they try to spell words or say the alphabet using letters that they see. Play I Spy or try to find license plates from all 50 states. Go around in a circle (or back and forth up the rows of seats) and call out a list of words in alphabetical order: aardvark; bear; cat; donkey; elephant. You get the picture.
2. Travel games are amazing. We had a Travel Yahtzee that was a blast in the car.
3. Coloring books or pads of paper. Bring small baggies or cups of crayons, pencils, markers, etc. for each child. If they have a cup holder, they can put the coloring utensils in there. This prevents scattering. And larger books can be used as improv. tables.
4. Books on Tape (or CD). Plug in a few good stories to keep the kids happily quiet and your nerves a bit softer. Check with your local library for a selection of children's tales. Or even try and download a few onto your handy-dandy iPod or MP3 player.
5. In this modern world, it is more and more easy to entertain children by plugging them in to a movie or tv show. I don't disapprove of this method (although I don't recommend using it the entire trip.) Whether you have a tv in your car or a portable DVD player, using a movie as a means of securing a few hours of relaxation time in peace and quiet can be very rewarding. It also works as a threat, i.e. if y'all don't settle down, there will be no movie.
I'd love to hear your childhood stories or your family tales and tricks for whiling away the long hours in the car. Or on a plane. Or a train. Or a boat. (Are there any more modes of mass transportation?) Big or small, I'm sure any helpful suggestions would be welcome. Good luck to all of you planning your next family vacation to sunny Spain (or maybe just Florida). A bit of forethought and preparation saves a lot of whining and groaning later.