Our guild had a challenge to use up scraps. People would bag up scraps from a completed quilt in a large ziploc bag and bring it to guild. They would turn their bag in and receive another person's bag of scraps. You could add in other fabrics, but you needed to use scraps from the bag you got. When the quilts were shown at guild, first the original quilt was shown, then the one using scraps was shown. It was amazing how different the quilts were and also how beautiful!
At a quilt show I attended last year, the local guild had a mini quilt challenge that was fun to see.