Mandala means 'circle' or 'completion'. When somebody creates a mandala it becomes a window for exploring one's inner self. Mandalas allow the creator to capture true self-expression. It is both therapeutic and symbolic. Art therapy is not about the end result but the process of achieving it.
Here is a PDF of Mandala designs, you can download and print for art therapy purpose. I personally recommend using colour pencils for a better experience.