DDad and I go to the World every December. I personally would not go to the last party and would rather go to the earlier one next year. But that's just me. However, a party ticket will get you into the park at 4PM and could be a nice day.
Here's some things we like to do:
-Take a sleigh ride over at Fort Wilderness
-Ride the monorail loop and get off at each of the resorts to enjoy the decorations, have a cocktail or get a bite to eat. Do some shopping. Enjoy the immensity of the Grand Floridian lobby while listening to the piano and/or brass choir. End at the Contemporary and go to the Top of the World Lounge for the fireworks. Maybe even meet up with a Groupie or 2

-Wilderness Lodge will have evening entertainment in the lobby: choirs, bell choirs Sweet Adeline's one year. I believe the other resorts do as well. This always puts me in the Christmas spirit. Eat some gingerbread.
After I have my coffee, I'm sure I can think of some more.