Mickey's Very Merry Christmas Party in the Magic Kingdom is an extra cost event. As such, it offers a wide variety of entertainment that you won't see in the park during the day (and don't forget the free cookies and hot cocoa).
There are major spectacles like the Holiday Wishes fireworks show, which is preceded by the projection show on Cinderella Castle, and the Christmas parade, as well as interactive fun at the dance party with Woody and Jessie in the Golden Horseshow and a cute Stitch-themed stage show in Tomorrowland.
However, one of my favorite shows is the performance by Rusty Cutlass, an all-pirate band that can be found on-stage in an appropriate spot across from Pirates of the Caribbean. They sing Christmas carols with a special buccaneer-based twist and even perform with hand bells.
Click the video accompanying this article to see Rusty Cutlass perform. You can catch them in person during the party, so be sure to keep an eye out for them when you're in Adventureland.