Thank you, and I’ll be happy to try to help. But first I have to mention that ALL of Europe is quite cold in December and there are no beaches that are anywhere near warm enough to even spend time on, much less go swimming. The only place that could possibly fit the bill is the Canary Islands, which is off the northern coast of Africa although technically part of Spain. It’s reasonably warm there during December, and there is quite a bit to do.
Phuket is incredibly popular among Europeans in December, especially the final week, of course, so it's critical to book early to get a good price at a beachfront hotel if that's your mission. It's also important to mention that Phuket is an island with more than a dozen different beach towns, each with its own personality and price range. Some are raucous and down-market while others are quiet and very expensive.
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.