I went to Florence for a 'big' birthday in December.. 5days.. We could have stayed longer there was so much to see.. It rained a couple of days but when the sun came out it was really quite warm.. The locals were complaining it was cold ,
I am going to add; Copenhagen, Prague , Venice, Iceland, Edinburgh, Paris, Marrakesh, Amsterdam, Egypt, find somewhere where there are things that you like doing and seeing. If your budget stretches you could do a 2 centre holiday or a cruise. Perhaps time your visit when there is a festival or market.