Since flights to Mexico City from the US and Canada are usually quite cheap this time of year as well, this could be a perfect short cultural holiday. This city has great shopping, dining, and nightlife, and the central historic and tourist districts are far safer than most people would expect. There are ruins just out of town as well, so a bit of everything is available.
If you decide to go off keto whilst you’re on holidays that’s ok. There are two ways to approach this. Give yourself a cheat day or cheat meal or go totally off for the entire holiday. I would prefer a cheat meal over a cheat day or cheat holiday simply because I know that one meal, while it may switch me from burning ketones to burning sugar, won’t make me feel too bad. I know if I have a cheat day or a whole cheat holiday then I feel sluggish, tired and won’t enjoy myself. It’s a personal choice but if you do decide to leave keto at home while you are on vacation don’t berate yourself for doing so. You’ve got a solid foundation on keto, especially if you’ve been keto for a while, so switching back on when you get home won’t be too painful. However, if you think that it will be a slippery slope to a total carbfest then maybe try to stick with keto. It can be done! If you have any tips or tricks for staying keto on vacation make sure to leave it in the comments. Knowledge is power and the more we share the better we get!
I think the best place for what you want is Thailand. You can choose from Phuket or Ko Samui or one of many other islands, and they all have nice beaches and good nightlife at a very reasonable price. There are other places in Asia that have warm December weather, but Thailand’s tourist infrastructure is better than the others so it’s just much easier to get around and do what you want. Let me know if you have any other questions. -Roger
While Cancun has become the king of the Mexican beach resort towns in the few decades since it was formed, it's only one of many, and all of them on both coasts have very nice weather in December. In fact, Cancun is probably the most expensive of them, so especially if you are coming from the west coast you are better off in Puerto Vallarta, Acapulco, or Los Cabos instead.
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.
If those flights are too expensive you might think about going somewhere else in Mexico or to Costa Rica. The area around Cancun including Playa del Carmen and Tulum and Cozumel has a LOT going on. And if you’ve been there you could instead go to the Puerto Vallarta area, which is very different and also great. For activities though it’s hard to beat Costa Rica. I think they invented zip-lining and the country is about one-third national parks with all kinds of fun activities. Let me know if you have any other questions. -Roger
I’m sure you’ll have a great time. I don’t have any good suggestions for where to celebrate Christmas, but I’m sure you’ll have no problems finding something by just walking around or checking online. The top draw in Barcelona is the architecture, and especially the 20th Century buildings by Gaudi. I recommend the hop-on, hop-off bus tour because it allows you to see almost all of the famous buildings from the street in just a few hours. Park Guell is worth a visit, but of course the main attraction is the Sagrada Familia cathedral. Check opening times and reserve a ticket in advance if you can.
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