We also go to Disney around the same time for DD birthday, and while it seems to be getting more and more crowded as each year passes, we are still able to walk right onto rides. Typically, the busiest rides seems to be the park favorites (Dumbo, Peter Pan and Winnie the Pooh), but then we walked right into the haunted mansion, BTMRR, and It's a small world. What's nice about December is when you are tired of the parks, you can hop the monorail to the GF and check out the enormous gingerbread house. Most of the resorts have a decorated tree in the theme of that resort, as well as carolers and such. Hope you have a great time, we always do!!