I just finished my weekly writing challenge. A group of us are assigned a prompt phrase and we then have a week to write something that includes that prompt. I particularly liked what I wrote this week. Even though we have a week I don't prepare for it or plot anything out. I just sit and start writing and see what comes out. When 'finished' I do only very light editing.
For me I like the idea of this sort of improv writing. I try different styles or focus on a different area of my writing that I think could use improving, being more descriptive or using more natural dialogue. While I spend most of my time working on my novel, this lets me branch out and play with things that I can then bring back to my main work.
Do any of you do this sort of thing or do you just focus on your one main project?