Actually, most of the popular European cities have fairly mild winters, in that they don’t spend much time below freezing and snowstorms are rare as well. It’s hard to make suggestions without knowing about your tastes, but I will tell you that the most popular European cities are London, Paris, and Rome, and none of them get snowed in often. If you are going to Italy, you should also go to Florence and maybe Venice as well.
My partner and I (in our 20s) are thinking about where to travel next December. We are from Australia, and since our last 2 trips have been to cold places (US and Japan) we are looking for a warm place to head to next. We aren’t into partying and I would love somewhere with nice beaches, wildlife and rainforests, but my partner can get easily bored so we need some activities and civilisation too. Probably looking at around 2-4 weeks depending on how much there is to see and do and would like to spend $100-200 AUD per night accommodation. Also, would like to feel safe wherever we end up.
Southeast Asia is easier to deal with, and there are cheap flights connecting in the Middle East and landing in Bangkok, Kuala Lumpur, and Singapore. Each of those is a very interesting city, and there are many smaller places to go in the same region. In Malaysia there is Penang and Melaka. In Thailand there is Chiang Mai and quite a few others in that area. In Laos you can visit Luang Prabang, which sounds like it could be a winner for you. And there is also Siem Reap, which is home to the amazing Angkor Wat temple complex.