To mimic stone, styrofoam is always good, you can cover it in plaster of paris and make a tomb or tombstone for instance. I'm not very good at the detailed stuff like moss or dead leaves, but some of that stuff you can use scenic debris made for railroad modeling. You can also use the real thing, like find some real dead leaves and crumble them up to look like 1/6 scale leaves. To make a dead tree you can simply take some cool looking tree brances or twigs and glue them together.