Resort hopping is the best during Christmas time. Each resort has their own themed decorations. Grand Floridian has a life-size gingerbread house with an actual store inside. You can't go inside, but the cast members are inside with a cash register. Beach Club has a gingerbread carousel and Yacht Club has a Christmas village on display. The Christmas trees at GF, WL, AKL are very tall and impressive.
Other than that, Main Street in Magic Kingdom is amazing during Christmas. Especially when Cinderella's castle is lit up and covered in the icicle lights.