Here's a little game I would play with my kids when they were old enough to scribble. My sister and I would play it when we were young and we evolved some rules along the way. We would take turns with the first person making a scribble. The second person had to make something out of it that included the entire scribble or as much as one could to make a picture from it. You could turn the scribble around for a better angle and you could add lines but it was preferred to use the scribble as is.
These are some samples I did just for fun.