Hey Amy!
I LOVE surprising the kids with something as exciting as going to Disney & meeting the Princesses!
One of my top favorite 'letting the cat out of the bag' methods is sending a letter. My kids are thrilled when there is something in the mailbox for them. A great big pink envelope with some glitter, fancy writing and stickers that holds a special Royal Invitation from Cinderella to your little Princess. An attached rose, wand or balloon adds to the fun. You can read the Royal Invite in a great theatrical voice.
Taking that theme a bit farther, you could wrap up a big box "From Cinderella" with a lid that lifts off easily. Have a balloon in there that will float up when she lifts off the lid with the Royal Invite attached to the balloon ribbon...Don't forget the confetti! You can make your own Mickey Mouse confetti with a paper punch from the craft store.
Or, while reading her favorite Princess bedtime story you can stop mid line and say "Oh! How amazing! Cinderella says here that YOU must come to Disney World to see her Happily Ever After for yourself! I guess we can't tell Cinderella no, can we? How soon can you be ready to go?"
Have a Magical time!