I would love to have a henna session with a lover. Or even washable marker, if semi-lasting designs are a bad idea. I've always been fascinated with henna, and there are some designs that would look AMAZING if you had the whole body as a canvas. (Think bold tribal designs cupping a woman's breasts and swirling down around her hips and hipbones, or tiger stripes along the ribs).
Or, if you wanted to be funny, handprints on the ass. Or a "___ WAS HERE" graffiti.