YouTuber IncredibleScience has a great at-home science project that’s kid-friendly: making waterproof sand.
All you need is a tray or baking pan lined with paper or plastic, some colored sand, and some Scotchgard or equivalent, and about half an hour in a ventilated area.
PROTIP: In the video, he mixes two colors in a glass of water, but once that happens, it’s pretty hard to separate them.
Previously: Waterproof sand won’t get wet (2008)