Hi, Dawn! Unfortunately, unless you are staying at a Walt Disney World resort, you cannot purchase the dining plan for your stay. The dining plan must be added in advance of your arrival, and can only be purchased by Walt Disney World resort guests. If you are hoping to keep on a meal time budget, I suggest a visit to
www.allears.net and then click on the Dining tab. You'll find a listing of the restaurants and their menus, including pricing. While this is subject to change, you can get an idea of what meals you might wish to enjoy and how much they will cost. The list includes things like snack locations and counter service meals as well.
Once you are in a Walt Disney World theme park, all of the rides, shows, fireworks, and other attractions are included in the price of admission. The only things I can imagine you might have to pay for on top of the admission ticket are souvenirs, food, drinks, You'll find one area at Disney's Animal Kingdom that has carnival type games, and those are extra as well.
I hope you enjoy every minute of your Christmas vacation at Walt Disney World!