Make your own autograph book. We did this when me daughter was little. Get a book from the dollar store with plan cover. You can either get a piece of disney fabric or use sticker and glitter pens. My daughter put stickers on the pages too. This was yrs ago so stickers where limited. Today you can get really nice stickers at craft store. Mickey will like signing his name on a page all fixed up with his stickers. Plus it will give child something to do before trip. Good idea for a plane project or car trip.
Another cute idea is after trip put pictures in the book of Mickey signing the book.