Hello, Angela,
You have already come up with two great ideas for your mid-day arrival in December.
Mickey's Very Merry Christmas Party is so much fun and could be a perfect way to begin your Walt Disney World vacation; however, the tickets are nonrefundable so I would be worried that if something were to happen to significantly delay your flight so you'd not be able to fully enjoy your evening. Since the party is offered again on December 15, 17, and 18, you may prefer to go later in your trip.
Disney Springs (formerly Downtown Disney) is a another fun option because you can come and go as your schedule permits. With all of the shopping and dining options, there is plenty to enjoy for the entire family. You could even spend the afternoon there then go to Disney's Grand Floridian Resort or Disney's Wilderness Lodge for the evening to enjoy all of the holiday decorations. In fact, you could make an evening of visiting all of the Magic Kingdom resorts. You can take a bus from Disney Springs to Disney's Wilderness Lodge resort. Once done enjoying its decorations, you can take a boat to Disney's Contemporary Resort where you can ride the monorail to Disney's Polynesian Village Resort and Disney's Grand Floridian Resort. You could even enjoy the fireworks from the beach at either of the two latter resorts. If you are staying at a different Walt Disney World resort, you can go to the Magic Kingdom to catch a bus back to your home resort.
I hope this gives you a few ideas of how to spend your first day at Walt Disney World. Wishing you an unforgettable trip! Please let us know if we can help with further questions. -
Kara