I see idling most frequently in the winter, when a driver is waiting for someone to get out of their house and into the car. Car-pooling is good, but idling is bad!
Creative ways to get people to stop idling in this scenario might be:
1. Stop the car, get out, go inside and see if there's anything you can do to help the person you're waiting for get ready to go! (It may hurry them along, too!)
2. Find a song on the radio that you like. Turn off the car and sing to the song. You can't turn the car back on until the song is over! (Challenge: wait for two+ songs, as long as it's not too cold to do so.)
3. Wear extra layers so that you're not as cold when you turn the car off.
#1 is my favorite because it's the least cold! And it involves personal interaction, instead of sitting alone in the car. Although #2 can be fun too, it can also get cold in the winter months!