performances at random, as long as the car is moving.
A few days ago the kids had bottled water in the car, and they discovered that if you hold the bottle outside the car window you can make noise with it, just like blowing over the top. They even adjusted the water level in the bottles to achieve an interesting blend of notes, and were putting the bottles in and out of the air to make "music."
I took the following photos with the car sitting in the driveway, but this will show you the proper angle to achieve if you want to try this:
If you try this, let me know how it worked out!