I would suggest batteries for your camera, nothing worse than having a great shot and can`t take it. Memories are very important, so you can keep reliving your vacation.
Also, I bring a sharpie for autographs, with some of the characters Big hands, it`s easier for them.
Something we`ve done in the past at Universal, so I`m not 100% they have at WDW, is to buy one of the refillable mugs with a handle. We attached it to the strap of our backpack, and we use the same one. I found that everyone will refill it free with ice and water, which is what we prefer anyway.