I like this one a lot:
http://lizmakesstuff.wordpress.com/2...g-almost-done/
I also honestly like when 4" squares are just pieced together with nothing in between. It makes everything blend together, which I think makes spotting them harder.
Like this one:
http://3.bp.blogspot.com/_jePxvJaxjJ...-Spy+Quilt.jpg
I like this one- but the sashing makes the spy blocks easier to differentiate:
http://www.quilt-lovers-guide.com/images/ISpyQuilt.jpg
The piecing isn't a challenge though, but you want a "spying" challenge- right?
For a piecing challenge- I like the tumbling blocks:
http://www.lynsfineneedlework.com.au...py%20Quilt.jpg