The guild challenge has been one of my favorite things about going to quilt guild!
We did the magazine challenge. Everyone was given a magazine as they came into the room. Then someone drew a number out of a bag, and we had to make a quilt that represented whatever was on that page.
Another year, I did the "Master's Challenge." I let everyone choose a card with a famous painting. I used the game "Masterpiece," for most of the cards, but printed more from what I found on the internet. Some examples are The Monalisa, Starry Night, Pink Girl and Blue Boy, American Gothic, etc. Here's a link with some famous paintings.
http://www.madisonartshop.com/20-mos...-all-time.html
Another year, we were given a bag with two coordinating fabrics and had to make a quilt that used both fabrics and contained at least 24 4-patches.