Make sure everyone brings plenty of water (or you might ask the people running the Ren Faire if they will supply bottled water, especially if your troupe is a part of their advertised shows). Also, bring and use plenty of sunscreen. Being outdoors for hours will turn y'all red as lobsters. You might also bring packable snacks, camp chairs, parasols, fans (either the pretty hand fans, or the small personal, battery-operated ones), and small gym towels, as well as the regular Bellydance Emergency Kit (safety pins, extra costume/makeup/props/whatever), and MONEY. Plus, if they want pics and/or videos, a camera, camcorder, or smart phone with a built-in camera, with extra batteries, charger, and or tripod.
If your Ren Faire is anything like ours, there will be vendor booths that offer bellydance-related items---just be aware that most of it will be that cheap airport stuff, but sometimes you'll run into something WONDERFUL.
