Oobleck (cornstarch + water)

In a bowl, mix about 1 cup cornstarch with about ½ cup water. Stir to the consistency where it's solid when pressed and liquid when scooped. Pour onto a tray.

Push your finger in slowly — it goes in. Slap the surface — it's hard. Pick some up; it dribbles.

Disbelief. Repeated slapping. Squeezing into a ball that "melts" in the palm. This is the most reliably 20-minutes-of-silence activity in the entire list.

Drop a small toy in. Add color. Push a fork through. Try to roll a ball that stays solid (you can't).