Hi, Beth!
Wow! A last-minute trip to Walt Disney World is such a fantastically magical surprise! You must be so excited!
Regarding obtaining an Advanced Dining Reservation at the ever-popular Cinderella's Royal Table, don't lose faith just yet; you still have some options.
First, look for an ADR for Princess Storybook Dining at
Akershus Royal Banquet Hall, located in Epcot's Norway Pavilion. It's not only a beautiful location with delicious food, but you'll also be able to meet several Disney Princesses as well; you'll even be given a keepsake photo with one of the princesses (currently Belle)!
Other options include
Cinderella's Happily Ever After Dinner, at Disney's Grand Floridian 1900 Park Fare. There, you'll be the Guests of Cinderella and Prince Charming, and are sure to love the elegance of the location as well as the wonderful food options.
Some Guests have been successful in obtaining a hard-to-find reservation by calling (407) WDW-DINE each day (including once their vacation in WDW has begun) just to see if there has been a cancellation. Other Guests' plans change, so when those changes result in the cancellation of an ADR, that reservation has to go to somebody...why not you? In addition, get to Magic Kingdom first thing one morning and head straight to the check-in area of Cinderella Castle. Ask the Cast Member there if there are any cancellations for that day; because you are there, in person, at such an early hour, the chances of your snagging one of those openings is much stronger than if you simply called Dining later in the day.
You never know; this last-minute trip may turn out to be the best one yet!
Send more questions as you think of them, too; we love to help!