I first heard about this on MFLresources before the summer holidays.  I thought I'd give it go in September.  It was so good I have done it with all my GCSE and Year 9 classes.  I have also done it for an observation that went well.  
I would like to thank Vincent at MFLresources for this fabulous idea, and have attached a photo of one the pupil books to give an idea of how it looks.  
snowball = error
tree = accurate
penguin = good 
penguin on skis = fab.  
I began by giving out very few trees to encourage good accuracy, then there seemed to be a fight over who would get the first penguin.  
To be honest the pupils seemed to really want to see my pathetic attempts at drawing a penguin on skis rather than wanting it for its own sake. 
Really positive lesson.