As for Costa Rica, I think it would be ideal for what you have in mind. It’s by far the most tourist-friendly (and English speaking friendly) country in that area, so it’s a great starting point. Belize is pretty easy as well, although there is far less nature to enjoy. Panama and Nicaragua are a bit cheaper and have similar things to offer, but Costa Rica has the best infrastructure and nature parks, so it’s less stressful for a first-timer. The snorkeling is better on the Caribbean coast, I believe, but the better tourist beaches are on the Pacific. The country is pretty quick to get across, so you could easily spend a few days on both sides, plus another few exploring the rain forests and volcanoes and such.
Christmas markets sprout up all over the country this month, and from the smallest towns to the biggest cities they’re a great place to mingle with locals, learn about regional handicrafts, and sample seasonal delicacies. Depending on the venue, you can find anything from medieval reenactments to outdoor concerts. But they’re usually at their prime once darkness has set in and the fairy lights are out in full force (which means there’s also time in the day for museum visits and city touring before indulging in the market flurry). Some travelers skip dinner altogether for a smorgasbord of yuletide samplings at the wooden market huts!
One reason that Vietnam isn’t as popular as it could be is that travelers often feel a bit hounded when they are there. For some reason, those in the travel business in Vietnam (working at travel agencies where you book tours and such) make tourists feel like they are being hustled. They are rarely 100% honest with what they tell you or about the prices they are charging. Still, it’s an amazing country and it’s also incredibly cheap, even if you end up paying a bit more than you were supposed to. I highly recommend it, and it won’t be a problem if you realize before you get there that you can’t take everything at face value in the way that you usually can in Thailand or Bali. Quality of hotels for the money is also very good.
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.