Rather than a mystery quilt, how about something like a brown bag exchange? First, you decide what your project is going to be and what your time frame is. It can be as small as a mug rug (1 week) or as large as a lap quilt (1 month); everyone just has to agree on the size and time frame. Then, everyone get the same bag (we would buy the $1 reusable grocery bags) and puts enough fabric to make the project. If it is a small to medium project, we would include the fabric to piece the top, and include the backing and binding, so we could complete it. If it was a lap quilt, we would just include the fabrics for the top. Everyone brings their bags to the meeting, and the bags are put together on a table or on the floor. Each member then picks a number from a basket. Starting with the member with #1, each member picks a bag. Before moving, she will look in the bag to make sure she isn't getting her own back. If she does, she puts it back and picks another. This continues until each member who brought a bag gets a bag. Then everyone takes their brown bag home and completes the project within the specified time frame. The mystery is that you don't know whose fabric you have and you don't know who has your fabric. I got a great baseball quilt top (for my son) and a lovely patriotic table runner when one of my bees did these exchanges.