For inspiration on design, I turned to mehndi patterns. Mehndi is an ancient indian art form in which intricate patterns using henna dye, are traced onto the palms and forearms of brides. It has become really popular amongst all cultures and there are now so many variations on the theme. It's possible to download patterns and designs from the internet, and re-create them using the black marker.
The more artistic you are, the more intricate a pattern you can attempt! :-)