if you don't already know them, writing.prompt.s on instagram/writing-prompt-s on tumblr have a really good list of prompts, here's some of their popular ones:
In the near future, you are making dinner because you are about to meet your girlfriend’s dad for the first time. All you have in the kitchen is cooked frozen steaks. In walks your girlfriend and her dad, Gordon Ramsey.
Write a story about how you became the world’s most powerfull person… by accident.
An old and homely grandmother accidentally summons a demon. She mistakes him for her gothic-phase teenage grandson and takes care of him. The demon decides to stay at his new home.
You're in charge of assigning every child on Earth the monster under the bed. One child in particular has caused every monster assigned to him/her to quit. You decide to assign yourself.
They say that whenever you shiver, it means somebody stepped on your past live's grave. The first human took their first step on Mars, and all of humanity just shivered.