@girls_can The mixture of cornflower and water is fun too. It has the weird property that you can put your hands in it and lift it up and it'll slide off, but if you move fast, it instantly hardens. I've seen a Uni "Open-day" demo with a bath of the stuff. You could jump on and off again quickly, but if too slow, you'd sink into the ooze.
No idea how someone thought this up, but putting it on a vibrating speaker gives odd results- https://www.youtube.com/watch?v=3zoTKXXNQIU