Some time ago on another music forum, I proposed a challenge to write for organ - to take a given modal theme and write up to 3 variations on it. The writing was judged on originality, playability, and (probably) my own bias towards what I thought was good. At the end of the competition, I put together the var. in what (I think) was a cohesive whole. The variations show a lot of creativity, and for many of the composers it was their first attempt at writing for the organ.