Really my first bit of advice is that the best place to get a really nice hotel that is on a beach or a hill and has the other nearby and also has great weather in December is Phuket, Thailand. You could go almost anywhere in southeast Asia in December and get good weather and low prices. But Phuket has over 1,000 hotels and hundreds of those are wonderful fairly luxurious hotels or villa complexes that could be perfect for a romantic honeymoon stay.
There's a massive range of quality and prices when it comes to hotels in Bangkok, and the city is enormous as well, so you really want to do your research for this place. Most people are happier paying a bit more to stay near the river and all the tourist attractions, or even in the infamous Khosan Road backpacker district, which is cheap and fun, if tacky.
Actually, all of my best suggestions for those things are in the article above. One challenge you’ll have is that all popular tropical destinations have their peak weeks at the end of December because so many people in cold areas have that time off. So flights to Central or South America are going to be fairly expensive, although buying soon can help. For hiking and views I think Costa Rica could be a good choice, or Nicaragua. You could also go to Bariloche in Argentina, though it’s probably not worth going all that way unless you were also going to spend some time in Buenos Aires and such.

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.
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