One thing I've seen a few people do on Twitter is post 3 words, ask for a sentence. You get some silly ones, but it actually makes you think and exercise your mind. Someone took that further and asked for a page of writing that incorporated 3 ideas.

I would it was an interesting way to force yourself to be creative while at the same time incorporating constraints. Just as you might have in a story you're writing. You can move in new directions, but you're constrained somewhat by what has already happened in the story.