It might be too young for her, but Guide to the Magic was a great book for our tweens. I loved the format with a picture and comments on each ride, and a place to note whether you rode it or not (with a sticker from the back of the book). There was space to keep a diary of each day (up to 14), to rate and comment on each ride, even to track expenses (more for parents). It even listed some hidden Mickeys to look for on many of the rides, and included scavenger hunts for each land. I thought it was a bit big to take to the parks, but I did see other kids with it, filling it out while in line. If she takes it to the park it has spots to get the nations from World Showcase passport stamps and spots for character autographs.
I just thought it was really neat, though you would need to look at one in person to decide if it was too young for your niece. I thought it was invaluable for prepping my guys for the next day's park...