A fairly standard writing prompt is to complete a sentence with your own idea. Such as:
“I wonder how…”
“The car turned around and…”
“The scariest animal is…”
“The book opened to the page with…”
“She read the tea leaves and…”
You can go on creating more and more of those. So do it! Completing the sentences is one thing, it’s just as creative to come up with the writing prompt as it is to answer them. Work on a list of ten and then see if you can come up with another ten. Pick your best and use it as a writing prompt yourself. How would you answer your own sentence?