Sri Lanka doesn’t really have a good infrastructure for budget travelers at this point. There are loads of nice hotels if you are willing to pay US$40 or more per night, but almost nothing below that except for a few hostels. I haven’t been to Palawan but I have been to Boracay and Palawan can’t be too much cheaper. Boracay doesn’t have many budget options either, so my guess is that Thailand or Cambodia is your best bet.
If your real goal is to sit on a European beach and not freeze or be alone in December, your only real choice is the Canary Islands. Tenerife is the largest of them, and the best one for English speakers. The average high in December there is 22C with 16C as a low, and in December it’s packed with Swedes and Brits and Germans who actually do sit on the beach or around pools, even though it’s a bit cool for most of us. It’s important to note though that the Canary Islands are technically part of Europe because they are a territory of Spain, but only the largest cities on each island are “European” at all. The beach resort areas are quite generic, and not very special aside from the pleasant weather and modest prices.

I’ve yet to do an African safari, although that is probable next year for me, and I know quite a bit about it from reading and publishing articles on them. That certainly would be a Wow thing to do. They tend to last around 5 days, so you could do that in conjunction with a visit to South Africa itself. That is another that I’ve yet to reach, but people rave about Cape Town and many other places in that country.
Turkey has bars all over Istanbul and the other tourist cities, but there are also cafes and tea shops and that sort of thing that are filled with locals (and some tourists) all evening. In Dubai and Abu Dhabi, the only bars are attached to hotels (and there are a LOT of them), but almost no locals drink, so there are unlimited evening alternatives. Other nearby Muslim countries such as Oman and Bahrain are similar, except with far fewer tourists (and drinkers).
And as you mention, Bali could actually work for you. I’ve spent two Decembers in Bali, and December is quite a bit rainier on average. Still I had a great time and it was sunny most of the days. The rain in the tropics (and Bali is almost ON the equator) tends to come down in 30-minute bursts rather than drizzling all day, so it’s usually pretty easy to avoid. The best part of Bali is there are loads of things to see and do, and the nightlife is excellent. The Kuta Beach area is fun for at least a few days, but I wouldn’t spend too much time there. You should also spend a few days in Ubud, which is very touristy but also interesting. And you could spend some time in Lovina, which is along the northern coast and it has all of the charm of Bali from 20 years ago before most of it got overbuilt. With all the temples and other local attractions, there is always something interesting to do, which is not true of many other places with great beaches and nightlife. Let me know if you have any other questions. -Roger
From exquisitely decorated storefronts to the amazing Rockettes Christmas show at Radio City Music Hall, New York City attracts thousands of tourists every December. Visit the Big Apple during the holidays! It’s a sight to behold. We recommend visiting Rockefeller Center to go ice skating, go shopping at new pop-up shops or just enjoy a cup of hot chocolate at Serendipity 3.

And you mention that you’ve been to Cancun and Yucatan, but just in case you haven’t been to Playa del Carmen or Cozumel, those could also be worth a look. They are both much more real tourist towns as opposed to Cancun, which is primarily a long strip of high-rise hotels and time-share buildings on a beach. I hope this helps. Let me know if you have any other questions. -Roger

If you are open to Africa you should consider Morocco. It will be a bit chilly in December, but still warm enough to enjoy everything. The culture there is fascinating and it’s also very tourist-friendly and safe. I think it would be cheaper for you to fly to Morocco than to the Americas, and the daily costs are very reasonable as well. Those should give you some things to think about, and feel free to follow up if you have other questions.
You mention New Zealand and of course it’s summer there then, so it would be a good choice. However, all of these places will be crowded and near peak hotel rates if they are warm. So I’ll be happy to give you advice if you tell me if you’d rather visit one or more European cities in the cold season, or if you are looking for a warm place like New Zealand where it will be fairly crowded. By the way, the southern European cities such as Rome and Barcelona are lively and enjoyable that time of year, as the Christmas season goes on, and there aren’t too many tourists except for a few that travel to see their families around the holidays. So it can be a good time for southern Europe, as long as you don’t mind wearing a jacket or jumper during the day, and maybe a bit more in the evening. Give me a bit more guidance and I’ll be happy to try to help. -Roger