We have a whole load of things we do for long trips. I use lots of ziplock bags for all the stuff and only give the kids 1 bag at a time. They can't get another one until a) they've returned the current bag, with all contents, and b) at least 1/2 hour has gone by. We always have a lap desk in the car too (the kind that is soft on one side and hard on the other)
Activities: 1) cut up pipe cleaners to different lengths - they can spend tons of time making things out of them (also leave a few full length in a "surprise bag" that they do not know about for when they "absolutely have to have" a really long one)
2) small size playdough containers - one at a time to be used on lap desk only
3) silly putty
4) small notebooks with twistable crayons (only 3 per bag) and a small sheet of stickers
5) I've thought about window markers, but haven't gotten any yet, but since they wipe off easily, if I can find some before our next trip I'm going to get them.
6) A new activity book, especially the kind with the yellow markers that you color over the spots and things "magically" appear.
7) a snack bag with things they usually don't get to have (ok, we usually have lots of these
8) felt dolls with felt doll clothes - like paper dolls, but put a piece of felt onto the lap desk and then all of the dolls and their clothing stick to it - different shapes would work too to "build" a town or some other picture on the board
9) books on tape/cd - we get about 10 of these from the library before any trip
10) sing a long tapes/cds - not all little kid songs, but also some music we like - then we each get a turn at picking what we listen to. (The driver always gets final say about whether or not we listen to anything!)
11) a couple of new reading books that either they get to pick out or are surprises for them
I have to admit, we usually have to spend about 5 minutes or so cleaning up at the end of each day in the car, but it is well worth it for the peace we get.
We also have to the rule that unless it is easily reachable for either me or DH, we do not stop to get anything that has been dropped.
Hope this helps - we've been traveling long distance with DD6 since she was 4 months old (the first trip was a 14 hour plane ride).
We also make sure that we stop every couple of hours to give them some exercise (we were told once at a rest area that this couple had seen people exercising their dogs at rest areas, but never their kids) We have short races and play follow the leader to use up some energy - it makes a huge difference, for all of us.
