One of the most magical aspects of Disney World is, of course, the Disney characters themselves- the princesses, Mickey’s gang of friends, and many others. Autograph books are very popular, and more creative guests and children bring clothing- such as cloth hats, t-shirts, even canvas shoes. Some bring stuffed animals, cloth character backpacks, or homemade autograph booklets. You can have them autograph posters, such as all princesses for example. Even more people use plain white paper and later add each one to a scrapbook with the photo of the character in question and happy guest next to them. More ambitious fans will use cloth scraps to make an autograph quilt, or a few have gotten the signature on their bodies for tattooing purposes. Of course, that’s a bit more extreme. 
Here’s a fun thought- why not have them sign postcards to send to friends and family (or to yourselves)?
Some tips for getting autographs include using big markers- many costumes don’t lend much flexibility for gripping pens or thinner markers- and bringing your own camera, film, and an extra memory card or two. Just in case. At the parks, even a one-use Kodak camera can cost an arm and a leg.
Small children can easily be frightened by characters, who are way bigger than they are and therefore can be rather intimidating and scary. For their sakes- and those of us waiting in line to take our own photos- don’t force them to meet the non-human characters! If little Billy or Anna is too scared, try your luck with the princesses and princes- they can relate more easily to them, and shorter people are often cast in those roles.
Please, don’t shove at costumed characters- who can fall over and hurt themselves or others- and remember that they don’t always have the best vision from inside the costumes. There’s a reason they have cast members following them around and helping them out. There was even a lawsuit after a child yanked at the back of a costume and the character turned around, accidentally hitting him on the head. (The suit was quickly dropped.)
Check with Guest Relations when you get to the parks for an updated schedule of character locations and times.