cityscape - I love the black line drawing against the richly colored skies. * I think I would add colorful water below to balance it and show the "colorfulness" of nature contrasted by the black and white of the city.
Hand painted tree rings - would recommend putting all tree rings on a board (which could be painted a coordinating color to add pop to the art piece. Really neat idea for after school kids. Might just do 9 rings in a square frame!