Just want to mention with the Sharpie pens it will wash out in the laundry. Might want to invest in fabric markers, you can get fat ones at the craft store. Also with a t-shirt bring a piece of cardboard to stick in the middle or it will bleed through.
My kids have been to Disney 3 times and they each have their own book, they use the same one each trip. We mark a page with the date of the new trip. They look through and see the characters from each year. We leave a spot on the opposite page for their pic with the character. Think we may have to spring for new books next trip since our neighbors saw so many more characters out and about in the parks. Make sure you put your name and address in each book, we lost ours and they were mailed back a month after our return.