There will be a henna artist at the Romani (Gypsy) Encampment at the Sioux City Riverssance Festival on October 2nd and 3rd. She is performing with the Desert Angels Belly Dance Troupe throughout the weekend, so make sure to head over between performances. Temporary henna tattoos can last 1-2 weeks after the henna is allowed to soak into the skin for several hours.
(Here is one of the designs that will be available.)
Also, check out the Gems and Jewels tent for belly dance costumes and accessories.