Take a story for children and change the ending. Is it going to be happy or sad? Is it going to be strange or funny?...
Here are some stories to choose from:

Goldilocks and the Three Bears
The Ugly Duckling
The Emperor's New Clothes
Sleeping Beauty
Puss in Boots
Little Red Riding Hood
Hansel and Gretel
Jack and the Beanstalk
Beauty and the Beast
Snow White and the Seven Dwarfs
The Enormous Turnip
The Princess and the Frog
The Boy who cried Wolf