I heard of this one recently ....
Greeting card challenge.
Purchase X number of greeting cards, any kind (birthday, sympathy - whatever - but get a mix of cards not all the same card). Give a card to each member in the challenge.
Each member has to take at least 3 elements from the greeting card to make a block, and only 2 of the elements can be color. Other than choosing two colors, they can use tulips if the card has tulips, they can use puppies if it has puppies, they can use the words - even in a twisty way (i.e. if the card is a belated birthday they can use a clock in the card). The possibilities are endless.
The completed block I saw was based on a humorous birthday card. The card had a picture of a plain white birthday cake on the front with a blue background and the words happy birthday in yellow. The inside read something about the fact that there were no candles on the cake due to a warning from the fire marshall. The resulting block was a very pretty layered appliqué flame using the yellow and adding other oranges/reds and set against a blue background.