If you are looking for first-rate destinations that are among the cheaper European cities, I’ll suggest Prague, Budapest, Krakow, and Berlin. You could visit all four of them by taking trains, or choose any one, two, or three. In December you can get quite a nice room in any of those cities, with Berlin being the most expensive by a bit. Food and attractions are also quite affordable in those cities.
Yes, I’ve been to all of the Nordic countries, but mostly during the summer months. However, I know that those Northern Lights tours are popular and well established. On one hand, you know you’ll be visiting areas where there is only a bit of sunlight in the middle of each day, so it won’t be great for general sightseeing. But the cold months are also the best Northern Lights months, and if you want to see them then it’s your best bet.
Mauritius would be an interesting option if you wanted to mainly stay in one place, which I think is typical on a honeymoon. Sri Lanka is fascinating and really nice, though most people who visit tend to tour around seeing some beach areas and some of the hills and jungles. So I’m not sure if it’s ideal if you want to just stay in one place. Kerala has some really nice resorts that are within easy reach of the backwater boat tours and other attractions.
If you are referring to days trips and that sort of thing, it will be much cheaper if you book once you get there. In Vietnam in particular it’s amazing how cheap tours can be if you book them through your hotel. I once did a 6-hour bus tour to Mỹ Sơn, which is near Hoi An, and I think I paid about US$6. It even included a boat ride and a basic lunch. That same day tour online would probably cost US$30 because the booking company takes a share and there is very little competition.