Take a fairytale, myth or story you know well. Alice in Wonderland, Prometheus, Cinderella, etc.

Take a genre/setting you've never written about. Cyberpunk. Steampunk. Space Opera. Bromance. Western. You can see where this is going. Preferably, pick something that is as unlike the story you've chosen as possible, because it'll force you to be creative instead of filling in the blanks.

You guessed it - write tab A in slot B. Hopefully, if they're different enough, having to fit the story into the genre/setting rules will mean things have to change - it stops being cinderella and turns into a story with cinderella-esque themes or tropes. 

Have fun.