Travel ideas for Europe at Christmas

Hi, I've got a flight booked to London on Christmas Day - but need inspiration on what to actually do! I am free to go anywhere in Europe. I plan to stay for 2-3 weeks. I've done Christmas before in Amsterdam and NY in Slovenia (it was freeeeeeezing). It's not the best season for walking around and sight seeing, but this is my only chance to be in Europe for a while. I know this post is a big vague, as I'm literally just trying to work out what to do.