I had a similar idea. My DGD had a princess Sofia book with Aurora in it. We are going to have them both sign that. I have also read about bringing a small frying pan for Rapunzel to sign.
I bought a spiral bound book of pastel colored index cards (unlined). The kids had characters sign cards that were color coordinated with their costumes. When we got home we included the signature cards in our photo scrapbooks.
You need to decide on a scrapbook item or a gift item. A book or pillowcase or other item is for a gift. A card, photo or other "flat" item will work for a scrapbook. I have a 9" stuffed Olaf I got for $9.99 for buying "Frozen" on DVD from the Disney Movie Club. I may bring that. But, it doesn't fit in a scrapbook or go with the pictures etc.
If you have the characters sign 5x8 white index cards with a click sharpie. You can then scan them when you get home, use your fave photo editing program (photoshop, pixelmator, what ever) to isolate the black, and then put it ontop of your photos of the characters.
Dd had a princess lunchbox signed by all 6 princesses that were on it. She had it done in kindergarten and still used it about half the time last school year (4th grade - she's outgrowing princesses but still loves animated and is liking disney channel movies now)
I liked it because I didn't have to schlep something every day to get signed. She brought it the day we are at Asherkus and had jasmine sign one evening we were in Epcot. so, I only put it in my bag for one evening and one day.