"Roundnet" was invented in the USA at the end of the 80s and is nowadays known as "Spikeball". The game can be played indoors as well as outdoors; it only requires a softball and a trampoline net.
In this game, two teams of two each try rule out the other party by playing the ball on the trampoline net so that the opposing team cannot play the ball back. The rules of the game are volleyball-like. Roundnet is fast, athletic, tactical, creative, coordinative, a lot of fun and can be played almost everywhere.