How about a small inexpensive photo album? If you don't have time now to do any printing of pictures, then you could take note cards for the characters to sign. When you get home slip that in one side and a picture of the character with your child on the other. Another thought is take a plain autograph book and buy some Disney stickers to dress it up.
Barb, I've been thinking about using your fabric square idea, but I don't want the kids fighting over it. Plus the last thing I need is another sewing or craft project. I'm leaning toward taking either totally plain T-shirts or sailor hats (dh is Navy, and we have a few of these in dress-up box) for autographs. My kids liked the extra attention characters gave them because of the elaborate books I made last time. I don't know if I want to cart those around again, though. I've tried suggesting no autographs - just pictures, but it's an idea whose time has not yet come.