My most indespensible thing is my backpack. The stuff in the pack is important too, but I'd be lost without my pack. (Of course, that's pretty much true of my life when I'm not at Disney too, but you get the idea.)
The two other things that are almost as indespensible are a little pocket knife/tool. I like the Leatherman Micra. It has scissors, a small blade (cause sometimes scissors just aren't what you need), small screw driver (for fixing glasses, opening compartments to replace batteries in my nephew's toys, etc), nail file, etc. The only think it doesn't have, that I wish it did was a can opener. But I rarely really need that, so most of the time, I'm okay.
The other thing is some duct tape. Useful for patching/mending things quickly, warding off blisters, labeling/identifying things, etc. Great stuff.