Yes I travelled a bit by train in southern India, I got lucky on my first few journeys with lack of people, then I hit some Indian holidays and it was hellish. I remember the trains being booked up for all the upgraded a/c and upper class carriages. One time even the Indians were fainting on the over packed carriage so you can imagine the heat for little old gringo me!
That said, I think going to Siem Reap in Cambodia would also be a great idea. And Vietnam is also really wonderful, especially that time of year. Hanoi is quite chilly in December and Saigon is still very warm. It’s worth visiting both of those along with Halong Bay near Hanoi and Hoi An in the center of the country on your way between them. You could easily spend 10 days in Vietnam or more.
Otherwise the big draws are the Great Barrier Reef, which is obviously focused on diving, snorkeling, and water activities, so it may not be too interesting, and Uluru, which is the big rock in the center of the country (and not near anything else of interest). There is also wine regions in the south near Sydney, and of course nature parks to see kangaroos and the other indigenous animals.
You might also consider Malaysia and perhaps a stop in Singapore. Malaysia is actually more modern than Thailand so everything is easy and reliable. They have cheap VIP buses that are very comfortable. Penang in the north is really nice and also interesting. Kuala Lumpur is a modern and pleasant big city. A bit south of that is the colonial town of Malacca, which is also very nice and worth a look. Going south from there you can reach Singapore in a few hours, and that is an amazing city, although accommodation there is expensive.
I think Spain is probably your best bet, and you can probably get there on a reasonably priced flight with a change in Dubai or Abu Dhabi. The winter weather is decent and the big cities are always packed with locals rather than so many tourists. Barcelona is probably more fun than Madrid, though both are big cities with a lot to see and notoriously good nightlife. If you get a cheap enough flight I don’t think you need to do a package. It’s pretty easy to get around Spain’s big cities just on English, as long as you do a bit of research. In 5 days you could spend 3 days in Barcelona and then 2 days in Madrid, or just 5 days in the Barcelona area. It’s a big city with plenty to see and some good day trips. You could also go to Valencia, which is also really fun and a short train ride away.
Unfortunately, it can be a bit complicated and time consuming to reach any of these Philippines resort areas. I don’t think I’d recommend doing more than 2 of them in a 8 to 10 day trip. The nice thing is that they are very secluded-feeling once you get there because they are so out of the way. Partly for that reason, I think staying for 4 or 5 days each is very nice.
I’ll be happy to give you some suggestions, but it would really help to know your starting point and approximate budget. At this moment I am writing this reply from Cancun, which could be perfect if you are in or near the US, but if you are in another part of the world there will likely be better choices. Although thinking about it, even if you are in Europe, a place like Cancun might still be best. Let me know and I’ll give you my best answer and an alternative or two. -Roger
Pretty much everywhere is going to be at or near capacity for those dates because half the world is on Christmas break. Between Australia and New Zealand, I think New Zealand would be far more interesting for 10 to 14 days. In Australia you pretty much have to spend a few days in Sydney, a few days in Melbourne, and a few days at the Great Barrier Reef, all of which require flights and will be very crowded over those weeks.
If you’re a bit more on the timid side, you can always use Yelp and TripAdvisor to get good insights on places people have visited. As a disclaimer, some of the ratings may not be accurate on these sites, so make sure you go through and read some of the more recent reviews that people have written. We did this for a few of the places we visited and had no issues.