First, list out every genre you can think of (mystery, science fiction, romance, etc.). Next list out every sub-genre you can think of; visit an online book seller if you’re having difficulty with this part.
Pick out two sub-genres of different genres; try to pick ones that are very specific and very disparate. Spend five minutes brainstorming new and interesting sub-genres that would combine these two sub-genres in some fashion.
Finally, outline a story set in such a genre. Concentrate on the things that are key to any good story: plot, character, story, drama, etc., not on the genre itself.
When you’re done, take a look at what you have. How did playing around with genre influence your story elements? How did it not? Did it inspire anything interesting? Was it difficult to constrain yourself to this narrow genre, or did it open up new possibilities?