Frisball

An inflatable ball can be converted into a disk if it is deflated, flattened, and the edges curled. A frisball is a ball or disk that is designed to convert from a ball or disk into a ball or disk, and back.

If a typical ball is deflated, one half can be pressed into the other half, so one is concave and the other convex. By following this procedure something like a bowl is produced.

That's fun too, but a Frisball is different because it has a rigid ring around its equator. By deflating the frisball, grasping the unyielding ring and curling it, the ball material is stretched under and over its frame until it is stretched flat into a disk.

The Frisball can thus be thrown like a disk or like a ball. If the Frisball is wound into a disk under spring tension, it can be thrown like a disk, to open up in mid flight, partially inflate, lose momentum and fall to the ground short of anyone catching it.

No wait, theres more, if the Frisball is deflated as it is curled, and air is not allowed to return, then the near vacuum inside the Frisball will keep it in disk form.