We do a mystery quilt every couple of years. One of our members finds a pattern (a free one on the internet) and figures out a way to do it in "parts". Each part is a clue and we get a clue every month for 4 months. We don't know how it's going to look until we get the last part sewn.
We have done "snappy bags" at another group I belong to. Everybody got their bags together in about 2 hours.