Hmmm, lens to go along with a 17-85... Personally, I think 85mm is about as much zoom as you will need for WDW in most instances. I have an 18-200 lens for my Nikon, but that is expensive, and I seldom use the 200mm end of the lens. I am not completely familiar with Canon, but I know Nikon has a nice 55-200mm lens--I would imagine Canon has a comparable lens, if you really want the zoom. If you plan on taking pictures on dark rides or of SpectroMagic, I'd recommend getting the 50mm f/1.8. It costs about $90 and is really nice. I would easily buy that lens over a zoom lens.
As for other accessories, if you plan on taking night pictures (besides of the parade), a tripod is a must. Comfortable camera strap (OP Tech is nice...Amazon for about $7), too. Let me know if you need any other suggestions.