Wow, I've never thought to do that! When I was a kid, my brother and I made funny signs to put in our van windows when cars went by. You can get really creative with it and hey, we're all just children at heart, so you can do that even if you don't have kids old enough to appreciate it. I wish I had some pictures!
"Headin' to the Mouse House", "Fantasyland or Bust", "Honk if you'd rather be on Space Mountain", or the mean but fun for a kid when you pass a bus: "Have fun at school, I'm going to Disneyworld!"