Venice can get pretty cold and they also have those famous Acqua Alta floods in winter, so read up on that before you go. But Rome and Munich should be easy to visit in December. One suggestion I will make is to consider visiting Rome for 3 or 4 nights and then base yourself down on Sorrento for the remaining 2 or 3 nights. Rome is one of those cities that is crowded and chaotic even in winter, to the point that just crossing the street can be frustrating, so many people get tired of that after they’ve seen the main sights. And Sorrento, on the other hand, is a lovely small city that is close to Amalfi, Pompeii, Naples, and the Isle of Capri, so it’s an ideal base to visit all of those. Sorrento is also cheaper than Rome, and a surprising number of locals speak English there, so it’s easy and pleasant. That time of year I’m not even sure if the tourist buses are running to Amalfi, although the local ones will be.
If you are more interested in nature and adventure than culture, then the top choice would be Costa Rica. The country has many beautiful national parks and it’s filled with things like zip-lining and canopy tours, just to name a few things. There are also volcanoes and surfing beaches and much more, plus a very good backpacker infrastructure of cheaper hotels and hostels.
The 2nd option is a bit early in the year. The Greek Islands are cold and mostly deserted until mid April or so, and the good weather really starts in May or June. This is true in Mallorca as well. The Canary Islands are much farther south so they have acceptable beach weather almost all year, although you might get some cooler days in March as well. Even still I wouldn’t really recommend them for a younger person. In my opinion, the Canary Islands are pretty boring because the majority of visitors that time of year will be retired people from northern Europe who just want to sit in the sun. The nightlife tends to be cheesy and/or weak if you can find it at all. Let me know if you have any other questions. -Roger
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.