Proj Rock! Mudpies, The Grownup Kind.
Have you ever made hypertufa? It's equal parts peat moss, cement, and sand. Add some water and you have some wonderful mud. You can make just about anything out of it...planters, water features, etc. It's porous, so it grows algae and moss nicely, depending on where you put it.
I'm making stepping stones. The molds are rings of cardboard. I line them with garbage bags, which gives them a smooth finish. Ideally, you have a level place to do it, but I don't. It's a crapshoot. Mostly I like mixing the cement.
I put rocks inside them. Hopefully they'll look cool when the cement dries. This is my first experiment with rocks, so I'm not getting too fancy with the design. These are used lava rocks from the barbecue, plus a couple I stole from the beach.
You dump the mud in, then slap it with the tool to get out the bubbles. It looks like jiggling fat! And check it, the two stones together look like a butt!