Think falls more under brainstorming than an actual prompt, but hey, it's something.
I've found that when I write essays, it actually does help to write an outline, although that's a far cry from a story/book.
So, what I did the other day was just write out what will happen in my story blow by blow. Something like this, just to give you an idea: "Lucy goes to the coffee shop. She sees a kid getting kidnapped. She runs after the car using her superspeed and peels the top off the car and saves the kid."
(This is not the plot to my book by the way, it's just what popped into my noggin first.)