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.
Unlike Bangkok, which has warm evenings and blazing hot days pretty much all year, Chiang Mai cools off nicely during December, at least in th evenings. This is an outdoor city with an emphasis on hiking and exploring temples so the cooler temperatures will be welcome for most people. It's still hot most days, and quite dry as well, so it balances nicely.
Another reason to consider Dubai is that the busy airport has cheap non-stop flights coming in from almost every major airport in Europe and Asia. As a result it's popular to book long weekend trips of 3 or 4 days to Dubai, where you can enjoy some sunshine and shopping before flying back home where it gets colder by the day. You might also consider stopping for a few days in Dubai and then moving onto somewhere else with cheap direct flights such as the Maldives.
Those countries are all very safe, and especially that time of year, you’ll be surrounded by dozens of other backpackers everywhere you go, including many solo ladies or girls in pairs. There is very little petty crime there, although it’s always wise to keep track of your belongings. It’ll be easy once you get there, and Bangkok is the perfect place to start. You can book a place near the famous “backpacker ghetto” of Khao San Road, which is quite affordable and really fun, as well as close to the top sights.
Unsurprisingly, the end of December is the beginning of the peak season for resort prices, so those who are able to come earlier in the month will be getting better deals. Still, the all-inclusive resorts are locked in heavy competition all year so this area can be a great deal compared to the alternatives, even in high season. If you are looking for where to go in December for an all-inclusive beach holiday, this is your best option for most people.