I've never tried this at WDW, but the one time I did use it, it worked great!
I don't know what these things are called, my mom found them at a craft fair. They're these rolled up bandanas with some kind of crystals or beads or something in the middle. When dry they lay flat, but if you soak them in icey cold water for a while, they swell up (soaking up the water). Then you can tie them around your neck, and the bandana stays cold for hours, and by extension, also keeps you cool.
I wish I knew what they were called, I'm sorry I don't have a better description. Maybe someone else here knows what I'm talking about. My mom found them at the Septemberfest craft fair in Schaumburg, IL.