Fun is a strange word that cannot be kept within any limits because it can often get out of control.
It can begin it can end it can be something spontaneous and something that is organized very carefully and not always, what is spontaneous is even worse than what is organized lol.
Yes we call the fun that comes out of nowhere a surprise although during organized fun we may be convinced that surprises await us but when you expect a surprise it is no longer a surprise.
