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.
You’ve been to Cancun, but have you been to Playa del Carmen? Personally I much prefer it because PdC is a lovely tourist town with hundreds of little hotels and restaurants and bars, unlike Cancun, which is mainly a long string of high-rise beach hotels that are so spread out that you are nearly trapped in your own hotel. It’s about an hour south of Cancun airport by taxi or shuttle. That would be another of your cheaper options, partly because flights into Cancun are pretty cheap. It will be the tail-end of the rainy season in all of these places, but that usually just means a 30-minute rain storm a few afternoons each week.
The rainy season is over so it's an ideal time to base yourself in Chiang Mai for some outdoor activities in the area. There are temples like crazy in the city center between the old walls, but even more interesting ones in the area just around Chiang Mai. There's also great trekking and such, available quite affordably from any travel agency in the city.
If you are very sensitive to humidity (I normally am, to be honest) then you’ll find more pleasant weather pretty much anywhere else in Southeast Asia in December. It can actually be cool at night in places like Chiang Mai that month. On the other hand, I LOVE Bali and I’ll probably spend all of next February there, which is still in the wet season. I’d still seriously think about Bali if I were you.
This year I’ve based myself in Miami, USA, primarily so I can visit many places in the Caribbean that I hadn’t been to before. I just got back from a week in Jamaica and soon I’m going to Puerto Rico, Dominican Republic, Panama, and Colombia, as mentioned. But in December I’m hitting the road again and I’ll be in Asia for the winter and probably in London for much of 2016. Sorry I don’t have more specific information to help you plan your trip, but it sounds like you’ve got some really good choices already under consideration. And I’m sure you’ll have a great time. -Roger