My top recommendation would be San Juan, Puerto Rico. It offers a wonderful mix of history and culture and nightlife and great food, alongside excellent beaches and abundant activities. Hotels and rental apartments are mostly in the mid-range for the Caribbean, but food and drinks and most other things are great value. You could get a place in the Condado area, which is close to the lovely Old San Juan neighborhood and plenty of nightlife all around. And then perhaps you could rent a car and stay for a few days at one of the smaller beach towns elsewhere on the island. You wouldn’t even need a passport, although it does feel fairly exotic while you are there. But because it’s part of the US, it’s also well organized and safe. Most people you’ll meet will speak at least some English, and many are fluent, so it’s easy even if you don’t speak Spanish.
Flights, on the other hand, can be pricey if you buy too early. My recommendation would be to book a resort soon and then put a fare-alert on the flights so you’ll get an email when the fare drops. For the Caribbean the cheapest fares are usually only 2 or 3 weeks out, and since that is a slow period you really shouldn’t have any problem (unless you are trying to go on popular Thanksgiving dates). Most likely the resort prices won’t change much as November approaches, so you could probably wait on that as well. There will definitely be empty rooms when you get there, so the resorts usually don’t start pushing up rates when they will be partly empty. Best of luck on this. -Roger
Based on your description, you definitely want to go to Costa Rica. The neighboring countries have many similarities, but Costa Rica is the shining star in Central America for tourism. They have endless national parks and nature sights, but also lovely beaches and adventure activities. The area in the north around the town of Tamarindo is the more luxurious part of the country, but there are many other beach towns on both coasts that could work for you. You will also probably want to spend one or more days in the Arenal volcano area, which has many other great activities. The Caribbean coast is more laid back and less family oriented, so better to focus on the Pacific. Have a great trip. -Roger

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.
As for your Europe trip, I highly recommend spending 3 nights in any city you visit. If you change cities every day or every other day you’ll end up spending most of your holiday on trains or in airports rather than seeing the places you flew all that way to see. So if Amsterdam and Barcelona are musts, I would add exactly one other city if you’ve only got 9 days. Unfortunately it’s not on your list, but the best and easiest one to add between them is Paris. There is a train from Amsterdam to Paris in a bit over 3 hours, and another high-speed train to Barcelona from Paris in 6 hours 27 minutes with no changes. Paris is easily one of the most impressive European cities for tourists, but if you’ve been there before and don’t want to go again you can obviously choose something else.
The next morning we hit up the hotel dining area around 7 am for a large breakfast before going out for our second day. They had gigantic breakfast bowls (Meal 3) as part of their offerings, so that’s what I got. I asked for a southwest bowl with no potatoes, and received a heaping plate of eggs, chopped ham, onion, green peppers, and melted cheese. I also got a side of bacon and had a fatty coffee that I made in my blender bottle with powdered coconut oil, liquid stevia, and half n half. I added some salt, pepper, and butter to my breakfast bowl and was very full. So full, in fact, that we did not eat again until 4 pm.
Trekking in Nepal during peak season might not sound like the most sensible choice for penny-pinching backpackers. But you can still enjoy the ideal weather without spending a fortune on in-demand hotels and expensive permits if you're happy to swap big-name trails, like Everest Base Camp and the Annapurna Circuit, for quieter treks to Kanchenjunga Base Camp and around the Langtang Valley. Idyllic conditions (dry but not witheringly hot) are also on offer in Rajasthan, one of India’s most vibrant and varied states. Chug between notable, attraction-rich cities like Jaipur, Jodhpur and Udaipur on an atmospheric and wallet-friendly Indian train, snacking on spicy pakoras and sipping sugary chai en route. 
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