We went the first week in Dec. last year. At CRT, Belle had a piece of ivy in her hair, but not at PSB. I believe Pluto had on a Christmas scarf at LTT. Crystal Palace- nothing I can think of. Chef Mickey's- nothing there either. I think the main thing you will find is a Christmas scarf here or there.
Don't worry- there is enough Christmas all over the parks to put you in the spirit!!! The characters out for meet and greets are often in their Christmas attire- Mickey and Minnie were both dressed for Christmas when we saw them, Pluto had on his Santa suit at Epcot, Donald had on a Christmas sweater at Epcot, etc. Have a magical time!!!