Me, I recently thought I would try a pure peat and Portlands topping and carve stone to look like a cow plop.
I never finished and now it's too hard to carve but it started out looking very promising.
If you can sell tufa dog poo, why not cow plop stepping stones?

Another one I got asked about recently was if I could do stepping stones that look like fancy man hole covers.
This kind of techno/metal look might appeal to a teen.

To wrap up you buy a plan stepping stone for a dollar and slap roughly 1/2 inch of wet tufa on the top and anything else you can imagine.

Or find free broken concrete chunks and cover those with tufa. Upright these chunks could make cool address markers for someones front yard.
they could be finished leaving a space for the address which could be filled in later.

There is no end to the ideas once you learn how to coat a chunk of old concrete with tufa.
And if there is a cheaper or more fun hobby I have yet to find it. It's good exercise and get's you out of the house.
And that Sunday drive becomes a lot more interesting when you are out on the hunt for that perfect chunk of broken concrete to make that Map of Canada stepping stone.:)

If the kid is artistic, there's also the possibility of making clay models and then making silicone (also be cautious of fumes) molds out of them either for the full mold or for the veneering part of it. Also mosaicing using stones, etc.