Use the power of X and Y to create a scrolling screen filler. Can you change the colours? Can you make the pattern appear across or up the screen, rather than down? Can you make a pattern as you draw? Can you change the size of the pen (and still cover the screen)?
Thanks to jordan1060 for the Pen Patterns and all the ideas on his wiki https://wiki.scratch.mit.edu/wiki/Pen_Art_Examples
