(1) Resting his left foot on the tube. There's only one tube, so it's difficult to call it a top tube.
(2) Resting both his feet on the hub of the rear wheel. He's not completely successful with this. He's also experimenting with dragging a foot along the side of the rear wheel.
I don't want to suggest that this is My Brilliant Child Inventing New Things because, in much the same way I discovered the law of the pendulum, someone else (in fact, judging by Bike Snob NYC, aka BSYNC, quite a lot of people) has already Been There Done That And Built an Annoying Subculture Around That.
Still. I can't imagine the population of 3 year olds doing this is all that large. Heck, the population of 4 year olds doing this is presumably not huge.