Looking for suggestions

My wife and I are looking to get her a small-ish tattoo somewhere visible like on her ankle. We are looking for ideas that people in the hotwife/swinger lifestyle would instantly reconginze so she can 'advertise' that she is avalible for sexual fun but vanilla people would just think it's a cute design. It would also be helpful to have a vanilla story to go with it. It doesn't need to be an original design, in fact so that it's reconginizable probably wouldn't be. Can you offer any suggestions?