You know you are dealing with a very special kind of game when people are inventing their own brand new pastimes within it.

This coincides with the whole player freedom and "unscripted awesome" discussion in which GTA V is so relevant right now.
When your game is so fluid and versatile that it allows for well rounded activities to be "invented" outside of established game mechanics, then you're doing open-world right. GTA V has been known for its excellence in these fields. Players have been entertaining themselves within the bounds of the game ever since release, rather than relying on the game to entertain them instead.