More experimentation today. The foam/ plaster combo gave me some pretty good looking rocks. Messed around with acrylic paint, spray adhesive, the wire foam cutter, and fake foliage for a mossy effect.
Foam seems to give me the best armature for a base, but what to use on top? I really like the look of the paper mache rock (the round one partially covered by moss.) I sprinkled foliage on it after I took these photos and really liked the result. I don't know if paper mache will make good carved stone though.... Might have to keep up with the plaster.
My best rock (plaster chip and acrylic paint)
Foam bridge, rocks, foliage/ moss
The round paper mache rock (hard to see in the pic, but it has a great texture)
My dining room table. Thanks to my housemates for putting up with this for a couple days....