Looking really good. Some clouds would be a really nice touch. Find a good resolution image of a sky and put the picture on a plane. Then put the plane in the background and then in the attribute editor there is a setting for cast shadows (turn it off) and a setting for receive shadows (turn it off).

Or you could create an image plane with your camera and adjust the settings on it.

Edit: Just make sure there are not too many clouds because your scene looks like a really nice sunny day, so the sky has to match that. To many clouds will make the scene look odd to the viewer whether they realize why or not.