Rubber door mats
A really nice thing you can cast is larger stepping stones using rubber doormats in a mold. I got hold of two different rubber mats with different sizes. I made one mold for each mat and used a steel mesh to reinforced the concrete because it is so big.
When casting, I filled one layer with concrete in the mold, then I put the steel mesh in and one more layer of concrete. I patted down the rubber mat up side down.
Then, I left it to harden for two days before I took apart the mold and removed the rubber mat. To get a nicer finish I used a rotating wire brush to get rid of the casting flash.