Click the Green Flag to start the program. Make a noise (you'll need a microphone on or attached to your device) to change the pen to a random colour. Experiment to see what makes a good effect - I found clapping worked quite well.
Thanks to getting unstuck day 6 for the inspiration to use the random block. Thanks to @mapastuszak for mentioning spirograph in a comment which inspired me to try and draw something that looks like circles. A next step for this project could be to change the pen size or vary the size of the circles.