My niece's rock collection turned me into a geology fan
I used to think her hobby was just picking up random stones. Seeing her identify each one with a guidebook showed me there's a whole system to it. How do you introduce geology to young relatives?
That driveway gravel idea is good because it's everywhere. After they spot the different colors, have them sort a handful by texture or which ones are shiny. It turns a basic look into a little detective game about how each piece got there.