A 3d printable surfboard that you can surf on air with in a car. pretty much like a finger skateboard but for air.

For it to work you need to go around 40 km/h or more but going more than 70 it will be kinda hard to use. (these numbers are not exact because it hasnt been tested at every speed, these are my best estimates)

It also has a hole in it so that you can attach a string and tie it to your hand so that you can catch it when you drop it.