In addition to the autograph book, we also bought each of our kids a fat pen as a souvenir. Our pen ran out of ink while Princess Aurora was signing my DD book and she told us to bring it to any store and they would replace it. Sure enough, the next store we went into they simply took our pen and gave us another.
My DD really wanted a pen with a cap, but we talked her into the one that clicks up and down so it would be easy to get ready for the characters and not have to worry about the cap. Not to mention, standing in line with a pen leads to many shirts with pen marks, so it's very easy to keep the pen in until they are ready to sign!
I hope you have a wonderful trip. It is very overwhelming and will be over before you know it, so do take some time to stop and just enjoy the magic!!
My final piece of advice would be....if you find you truly enjoy the music from any of the shows (parades, Wishes, etc.) buy the CD while you are there. We didn't our first trip and I regretted it. Now we buy one everytime and that is what we listen to in the car. Enjoy!!