Ways to affix tiles to tiles.

I have a lot of access to a lot of ceramic floor tiles.
The biggest are nominally 24" X 12".
By themselves they are not thick enough to build with or stand on.
I think stacking them Jenga style would fix that.
Back to back, I think thinset mortar will work great.
For making a 2'x2' stepping stone, I would want the smooth tops facing each other, so the rough surfaces could face the ground and the foot traffic.
I don't think thinset will work for that application, but I have no real experience.
I could rough up the smooth faces, but that's an added step that may not work.

Is there a cheap,water proof , low toxicity adhesive that might work better than thinset?

If the tops are glazed, I can't think of anything that will work and hold up over the long term subject to foot traffic. Unglazed surfaces, like the tops of terra cotta and quarry tile, might have enough roughness for thinset to key into. But even that seems iffy to me. I guess there are epoxies that might do the trick, but then you're introducing greater costs and toxic gick.

I think you are right about the possible alternatives to thinset.
I think I'll make a few squares and try to build a raised bed with them.

Maybe you could study up on how to do mosaic work.  Ancient Rome and Greece had some amazing works with a very simple color palette.  It might be fun to try something like that.  The tools needed are very simple and not too expensive!

I've used a number of 12x12" salvaged tiles in the garden to hold down cardboard mulch, mainly in the paths between rows. Mine are porcelain and easily stand up to being walked on, but I imagine that softer ones would work too if well supported.

The idea just wants me to grab some paints and nice polymer grout in blue grey, and also in tree trunk brown, and off white, like birchbark, and start painting like crazy!

Maybe donate to an artist - tile artist. They would likely be grateful gaga. If you have a wall they could put them onto - double happy