Someone on here posted a great idea, they had one of those poloroid sticker cameras and they took a picture of the child getting his autograph and then placed the picture next to the signature. Someone else I think used card board squares that would fit inside one of those small photo albums so that way they wouldn't have to worry about rain.
Others have said they use T-shirts you can't wear them just bring them. Especially if it is a special day or something they decorated them and then used it as a wall decoration. The one thing we do (not creative) we ask for 2 small bags that fit the book and keep the pen and book in that so it is easy to find it and stays clean.