Hello,
As I have posted elsewhere, I went on this trip last Christmas and had a marvellous time. Three tips:
If not already done, paying extra for lake view is very worthwhile - castle and lakeside are lit up at night
On included trip to island, be sure to try the drinking chocolate - like no other - marvellous!
Do try Bled cake as often as possible, your diets can wait.
Also, if you like chocolate,, there is a shop between the hotel and lake which sells salted chocolate. Sounds horrible, but is rather more-ish.
Have a great holiday.