Hi Anastasia,
What a lovely way to celebrate your 21st anniversary, with a trip to Walt Disney World!
My favorite place to watch the Magic Kingdom fireworks from that is not inside of the Magic Kingdom is the beach at Disney's Polynesian Resort. The view from there is front and center - you will be looking directly at Cinderella Castle. If you are also looking for a nice dinner to celebrate, you can view the fireworks from both Narcoossee's at Disney's Grand Floridian Resort and Spa, as well as from California Grill at Disney's Contemporary Resort. I really enjoy the food at both of these restaurants. For a special occasion, I would recommend California Grill. If you eat a meal at California Grill early in the evening, if you save your receipt, you can return to the restaurant to watch the fireworks from their observation decks.
As to something special at the resorts that evening, there are many
special events throughout the holiday season at Walt Disney World. If you do plan to be in the Magic Kingdom area, I might suggest a monorail ride around to the resorts to check out their beautiful decorations. Disney's Grand Floridian Resort and Spa even has a giant gingerbread house!
I have one more idea for you to make fireworks viewing extra special for you anniversary: a
fireworks cruise. I have been able to take a fireworks cruise to see the fireworks both at Epcot and Magic Kingdom. I really enjoyed each. A private boat, with your own captain, will take you on a tour of the waterways. Then, your boat will have a prime location to view the fireworks. This is truly a magical experience.
I hope that this gives you some ideas for your celebration, Anastasia! Have a great time planning, and please do not hesitate to ask if you have any further questions.