Experiment with a sketch that includes all of these:
One element controlled by the mouse.
One element that changes over time, independently of the mouse.
One element that is different every time you run the sketch.
Here is a pretty simple sketch showing a background that changes every time the code is run, with a 'pen' made from a circle that follows the user's mouse and changes size in every frame. The user can make a drawing on the canvas, but it will also be masked by an expanding color field with a very low alpha value of 1. This will allow any pre-existing 'pen strokes' to fade into the background.
It's pretty fun!
I had to reference the framework for the random() function, but I otherwise kept it pretty simple. This color masking idea could be really cool for intentional colors that combine to reveal a hidden message over time.
Side Question: How do you make an object 'bounce' back off the edge of the canvas?