The most important holiday in Japan is oshogatsu (New Year), and the month leading up to it is packed with fun festivals and markets. Standouts are Shimai Kobo (December 21) and Shimai Tenjin (December 25) both of which offer great ceramics and handicraft shopping, in addition to lots of good things to eat and drink. From December 8 through the 17th, thousands of lanterns will be placed around Arashiyama, including in the enchanting bamboo forest. If you’re there on New Year’s, you’ll catch the famous the bell-ringing ceremonies -- get up real early and see the monks at Kyoto’s largest temple, Chion-in, or drop by a small-scale local shrine instead.
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.
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.
Rio is probably the most beautiful city in the world as far as the setting is concerned, but it’s also famous for petty crime. As you mentioned, I think it’s quite easy to stay safe as long as you are aware of the situation. The biggest problem by far is tourists getting robbed by someone holding a knife or sometimes a gun, and they are often in groups. However, this almost always happens to people who are walking close to the water on the beaches after the crowds have gone for the day. The sidewalks along Copacabana and Ipanema are brightly lit and safe, but there is a mound of sand between the sidewalk and the water, so if you go to the water you can’t be seen by the security guards and police on the street. As long as you avoid that, and any other situation where you are alone in a dark place late at night, you’ll be fine.
Visit the Bahia Palace and gardens with its intricate artisan details. An overnight train ride north would bring you to the Moroccan seaport city of Tangier, once the playground for the rich and famous, celebrities, and artists. Admire the unique architectural and culinary influences from Spain, Portugal, France and Northern Africa as well as panoramic views of the city and bay from the hilltop Kasbah. The Musée la Kasbah is fascinating, and Café Hafa is a wonderful stop to quench your thirst and rest your feet. Visitors should visit Cape Spartel and the Caves of Hercules for its incredible history and captivating scenery. Perhaps a camel ride on a sandy beach or through the desert is your thing? Your Moroccan excursion is guaranteed to cover all bases from exotic to adventurous to mystical.
If you are going for one week I wouldn’t go all the way to Thailand, but if it’s two weeks I think it’s worth it. It takes most of a day in each direction, but flights are often reasonably priced, and everything there is cheap once you get there. I’ve actually lived in Thailand for 6 months and have visited many more times and it’s actually very easy and stress free once you are there. All the important signs are in English and nearly everyone you’ll encounter as a tourist will speak enough English to help you as well. You can get around by train or VIP bus for cheap, or fly to the islands for cheap. It’s very safe there as well. You could spend a few days in Bangkok and then head to one of the islands or take a train up to Chiang Mai.
Much of Mexico is ideal in December, after the rainy season has ended; if you go early in the month, you’ll avoid the peak holiday-season rates that kick in around December 18. On the Pacific coast, the landscapes are still ultra-tropical green, and the ocean is bathtub-warm (80s), with exceptional clarity for scuba diving and snorkeling. Mexico City is a smart place to spend the holidays, when the locals have headed to the beach and you’ll find much less traffic in this famously congested city. (Do keep in mind, though, that some of the more sought-after restaurants and galleries will be closed.)
Few people travel between Thanksgiving and Christmas—specifically, from the Tuesday after Thanksgiving to the Thursday or Friday before the peak Christmas rush—so you can enjoy substantial savings off peak-season rates (four nights for the price of three, seven nights for the price of five) and your chances of an upgrade are good—if you book through a Caribbean specialist with clout.