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.
Lace up your boots for an end-of-year adventure at the wild ends of the Earth. December is the ideal time to explore Patagonia’s mind-blowingly beautiful Torres del Paine National Park. Embark on an epic trek along the eight-day circuit, or take the four-day ‘W’ trek for Instagram-worthy views of Los Cuernos (the ‘horns’), the region’s iconic jagged granite peaks. Similarly breathtaking experiences can be had diving the coral reefs around Thailand’s Similan Islands. Beneath the sparkling surface of the ocean you’ll find sharks, barracuda, rays and green and hawksbill turtles – it’s Finding Nemo brought to life.
In Rome, the Coliseum and Ancient Rome are outdoors, but the city also doesn’t get as cold as the others so most likely you’ll be fine if you bundle up a bit. In Venice, they often have those floods during December, but they come and go every 6 hours so even if you get unlucky you’ll still be able to get around half the day, and it’s a small city anyway.
Southeast Asia is easier to deal with, and there are cheap flights connecting in the Middle East and landing in Bangkok, Kuala Lumpur, and Singapore. Each of those is a very interesting city, and there are many smaller places to go in the same region. In Malaysia there is Penang and Melaka. In Thailand there is Chiang Mai and quite a few others in that area. In Laos you can visit Luang Prabang, which sounds like it could be a winner for you. And there is also Siem Reap, which is home to the amazing Angkor Wat temple complex.