Thailand however should be fantastic in December. You can travel to Laos and Cambodia as well, which are also very interesting. If I were going on vacation in December that is exactly where I'd go.
The night boat from Phnom Penh to Siem Reap (town next to Angkor Wat) is an incredible trip. One of the most beautiful river valleys in the world. Serene, and though there's light from villages on the banks, it's still dark enough to really see some stars.
Plus, Cambodians are very liberal about cannabis. Esp in the north near Siem Reap. I don't know how many motorcycle taxi guys would run up to me while I was walking down the street smoking a self rolled cigarette asking if I wanted marijuana. (Answer: Normally, yes, but not right now. I just ate a bunch of happy food and I gotta go find a place a sit down while I can still walk.)