We thought the Animal Kingdom Lodge decorations were very beautiful and unusual. We are going to check out the Grand Floridian and Fort Wilderness Lodge decorations on our Christmas trip this year. I have heard they are great.
We will be staying at the Dolphin this Christmas, which has great decorations. We thought the Yatch Club decorations were kind of run of the mill when we stayed at the YC last time. Wasn't impressed with the Polynesian's decorations, but many people love them.
You may use the monorail to go to any of the resorts it visits. You may use the boat to go from the MK to the Fort Wilderness Lodge and the boat at Epcot/MGM, or walk, from the International Gateway to the resorts located at Epcot/MGM. The other resorts can be reached via the resort buses from any of the parks. They are intended for use by the WDW Resort guest, but no one checks for room keys or proof of where you are staying. The AKL is kind of remote so you may want to visit it from the AK park using its bus. We thought the Christmas decorations were nice but fairly normal at the Port Orleans Riverside when we dined at Boatwrights last Christmas. Where ever you visit, the decorations are themed and very extensive.
All of the resorts have security guards posted at the gates to the resort. For this reason, you may want to stick to Disney transportation when visiting. They check to see if you are on their Dining PS list (they will give you a 2-3 hour temporary permit) or guest list (they issue a length of stay parking permit if you are staying at the resort). I think they would turn you away if you said you were there just to look around. I'm sure they get a lot of that at Christmas.