Fun stuff
I have no idea why this is so much fun but give it a go. Grow (version 3).
And then you can try to escape from the Crimson Room
Since John just asked me to try and find the link to some chalk drawings I pointed him at a while ago (Maybe Ajay pointed me at them, I forget), here are links to Julian Beever and Kurt Wenner. I’m particularly impressed by Julian’s 3D illusion pictures.