After I got my Digital Rebel, I had to wait for a little financial adjustment (read: I was broke!) before I could get another lens, so I used the 18-55 kit lens on my trip to WDW in December.
The kit lens is actually vary nice, and I got some terrific pics with it, some of my best ever at WDW.
However, 55mm just wasn't enough zoom for me. I had lots of trouble getting close enough to details on the castle, and especially animals on Kilamanjaro Safari.
I have since bought the Sigma 18-125 that Kyle mentioned above. I don't know if I'd call it the "perfect" lens, but it sure fits my needs perfectly. It's small and light compared to my older Sigma 28-200 lens, takes sharp pics, and focuses fast. Cost is about $230.
But as for carrying the camera around in a backpack - how are you supposed to trake pictures if the camera is all wrapped up securely on your back? I carry mine around on a strap around my neck, leave it on all the time (with sleep mode enabled to save battery life), and still miss shots once in a while.
Carrying spare memory cards won't rewuire a backpack (they fit in your pocket), and I carry a 1-gallon Ziploc bag as a camera poncho in case of rain or water rides, so keeping the camera dry is not an issue.
You may be nervous about carrying $1000 worth of camera around on your neck, as I was the first time I took an SLR to WDW, but after the first day or so, you get so involved with taking pics that you forget to be nervous - just make sure you don't get so involved that you forget to be careful!