Here's a possibility:
http://quilting.about.com/od/Mystery...t-Patterns.htm
You could also do any of Bonnie Hunter's free quilts -
http://quiltville.blogspot.com/p/free-patterns.html
Scroll down to the bottom of the page. She has 3 available.
Also, how about any of the Mystery Train quilts right here on QB?
Good luck!