Whether you like the Disnified Happily Ever After versions or the grim Grimm originals, fairy stories are a great source of inspiration for a writer.
You can rewrite the tales with a modern twist, or a funny one, or you can simply take the morality-play form and use it for your own story.
I come back to this prompt idea again and again because it is such fertile ground and because EVERYONE knows a fairy story or folk tale (if you need a reminder of some, go here).
Alternatively, you could choose to write an allegory (think: Narnia, or Animal Farm). If you do write an allegorical story, however, bear in mind this advice from James Scott Bell’s book Plot & Structure:
“Allegory is difficult to do well, since it may just come off as merely preaching in the guise of an imaginative tale…Make the characters real and not just stand-ins for your ideas.”
Rewrite a fairytale/folk story or Get Allegorical