What's a good symbol for "Serenity"?

I've put a lot of thought into the meaning of my next tattoo. Ive decided on "Serenity" (from the serenity prayer" The problem is, I don't really want to get just the word tattooed on me. I'm looking for a symbol that means "serenity". I don't want to use a japanese charachter or anything. I want something as unique as possible. Any suggestions with a little background is appreciated.