Another great exercise from Twyla Tharp's The Creative Habit, but one that sounds suspiciously familiar:
Give Yourself a Little Challenge
George Harrison once decided, as a game, to write a song based on the first book he saw at his mother's house. Picking one up at random, he opened it and saw the phrase "gently weeps," whereupon he promptly wrote his first great song, "While my Guitar Gently Weeps."
You can give yourself the same kind of challenge whatever medium you work in: paint only in shades of green; write a story without using the verb "to be"; film a ten-minute scene nonstop with one camera. Giving yourself a handicap to overcome will force you to think in a new and slightly different way...
Turns out I've been doing this all along! That's exactly what each of these weekly prompts are—a handicap to overcome that forces me to think in a new way.
Even when the resulting story/play/cave painting/whatever is unrecognizable from the original phrase or idea I started with, I still would not have gotten where I did without that initial challenge to push against.