This is a huge question and I’d need to know more about your preferences and your budget to answer it properly. But I can give you a short version here. In my opinion, which is matched by many other people, the most interesting and dramatic cities in Europe are London, Paris, Amsterdam, Rome, and Venice. Visiting those would require about 16 days to do it properly because you’d want to spend 3 or 4 days in each city, except maybe only 2 days in Venice because it’s quite small.
November is a tricky month in tropical Asia, as you’ve discovered. In Thailand, Cambodia, and Vietnam, it can be rainy, and that includes some days where it rains the whole day rather than the quick cloudbursts you get in June and July. The best tourist beach in Vietnam is in a town called Nha Trang in the south, and there is Da Nang near Hoi An in the center of the country, but you’ll get November rains in both of them.
There’s really nothing like the holidays in Manhattan. December tends not to be too cold in New York -- you may actually pine a bit for snow as you mosey past Fifth Avenue shops, peering in at the elaborate holiday window displays. The rink at Rockefeller Center gets crowded, but catch it midweek and you’ll have room to skate a lap beneath the most famous Christmas tree in the world. And if you’re still picking up last-minute gifts, you really can’t beat the shopping in this town. Even usually gruff New Yorkers seem to have a brighter spirit this time of year, and carols echo through the subway stations thanks to the city’s many transit musicians. There’s no better time to go and bask in the Home Alone 2 Christmas vibes.
The Keto Diet works best if you can maintain it. If your body is continually switching from converting carbs to glucose, instead of converting fat to ketones for energy (which is the process of ketosis, wherein the liver converts fat cells into energy), your weight loss will possibly stall. Consequently, you may find that some health benefits of eating a Keto-friendly diet begin to diminish. Find a healthy balance on your next vacation; with some planning, you can maintain your Keto Diet even when you are out of your routine.
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.
Thank you, and I’ll be happy to try to help. But first I have to mention that ALL of Europe is quite cold in December and there are no beaches that are anywhere near warm enough to even spend time on, much less go swimming. The only place that could possibly fit the bill is the Canary Islands, which is off the northern coast of Africa although technically part of Spain. It’s reasonably warm there during December, and there is quite a bit to do.
In SE Asia you have many choices, but probably the best choice for a family would be Phuket, Thailand. It’s more expensive than Goa, but the infrastructure there is much better. Patong Beach is the biggest town on the island and the main tourist hub, but for a family I’d recommend Karon Beach or Kata Beach, both of which are just a bit south of Patong, and both lovely.
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.