My boys are getting these Toy Story photo albums that measure 4x6. Since they will only want certain autographs, we're going to get postcards for the characters they want and then only carry them with a pen into the parks. We'll have the characters sign their pictures.
The album will then become a memory book to hold other momentos, i.e. autographed postcards, menus, napkins, stickers, tickets and photos of their trip.
I thought of the T-shirt idea, but didn't want to lug them around the parks, and thought them being boys, they would get limited wear and then shoved into a drawer. This way everything will be in one small album for them to keep.