You can make an autograph book we easy enough check out the craft board here for some amazing pages you can print out onto card stock. there's adorable fake coupons kids can 'use' to get treats too! Sometimes it encourages characters to do special drawings in the books too.
I highly recommend using a sharpie for characters to sign. Pens are very thin and sometimes they can get gloved hands around them. Some characters like Buzz use stamps so be prepared for that.
Check out disney.go or laughingplace.com. there's 2 types of books a basic blue one and a pink princess one that holds pictures.