I approach it as two different problem sets.
1) The drive: I'm a big fan of the portable DVD for young kids. Once they get to reading age its value lessens, but for the younger set a long car ride is just a long car ride. They are not going to get the wonder of the endless wheat fields, or whatever else you're driving past.
2) In Camp: Less is more, as has been said all kids love water, and my kids are impervious to cold. A stick, a rock and a project can keep them busy for hours (lets make boats out of sticks and put them in the stream.......) More often than not I end up more involved then they do, but it keeps us all busy.
Edit- almost forgot, the second half of "less is more" is the emergency box of toys, games, etc.. for rain, wind, cold times in the tent. Keep it boxed up and hidden until you need it, will really shorten what would otherwise be a very long afternoon.
Most important comment I've seen so far: Make sure they are involved, and not just an annoyance. If you're going camping with the kids, then the purpose of that camping trip is: Kids camping. Nothing more. These are not the trips to practice survival skills, summit 14'ers, etc...
You can get kids sized folding camping chairs at Dicks sporting goods (and some grocery stores around here) for about $6. Went a long ways with both of my kids to let them know that they are part of the experience when they got chairs just like mom and dads. If you look long enough, you can even find them without Spongebob, Woody or Lightening McQueen (although I would totally have a Lightening McQueen chair if they made them in adult sizes).
-Matt