I’ve done most of my travel as a solo, and I’m now a couple years older than you, so I know exactly what you are asking about. If you had even 2 full weeks I’d suggest going to southeast Asia, but as you say, the travel time could be a killer for a shorter trip. Still, it’s a unique experience with a lot of positive aspects to it, so you should keep it in mind. If you are still considering it, I’d suggest flying to Bangkok for 4 days or so, and then spend the rest of your time on one of the islands such as Kho Samui or Phuket. If you want more information on that, let me know.
From December through February there are literally no European cities that could qualify as having “great weather,” except for the Canary Islands, which is listed below. So during the winter Europe is all about cultural tourism, and lower hotel prices make it especially appealing for those who like to stretch their travel budgets and avoid crowds at the same time.
For those from the Northern hemisphere in need of decent a blast of winter sun, the picturesque coastal town of Essaouira is the perfect place to put your feet up and relax. Fresh-caught fish sizzles on beachside grills, whitewashed houses with blue shutters lend a Mediterranean vibe, and you can lose hours wandering through the dozens of art galleries and workshops.
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.