Ideas:
-Make the moss darker. I don't think it fits the surrounding area
-Let parts of the forrest around the game "connect" with the actual game-field: make grass from the rocks on the side hang over the field (only a little, of course). let a root from a nearby tree stick into the game somewhere. Adjust the moss color to fit the surrounding colors.... that type of thing.
-the non-breakable stones don't convince me yet. I'd take a different model for those, and make them look like the stones on the sides of the field?
Just ideas...
But it looks really good already, well done!