Top coating readi made concrete is fun and easy because the hard work is done.
Small plain stepping stones of various shapes are very inexpensive.
The basics for top coating are Portland's cement, probably sand and a bottle of Elmers wood glue or a concrete adhesive.
Pigment's, paints, any thing that might be used for mosaic.
Peatmoss, glass, etc, the list goes on and on.
Artistic talent and an imagination and there is no limit to what you can make.
Marly has recently posted a picture of a beautiful topcoated stepping stone with stars and the moon on it.

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.:)

Please be sure to supervise the teen while he/she is mixing the cement mixtures and making sure that they're taking all of the proper precautions--respirator, gloves, etc. I know a lot of we "adults" have made our share of mistakes and end up with burns, cuts, etc., even after doing this for awhile. I would hate to think of a young person inhaling any of this into their lungs or maiming themselves at such a young age plus the fact that not all teenagers like to listen to advice and we all know they're invicible :0 Plus the fact, that if you're not his parent, you could be held liable for anything happening to him. But then again you might have a more mature teen and he/she will adapt to it.

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.