In December, the west coast of Thailand is perfect, with Phuket, Krabi, Khao Lak, Koh Lanta and Phi Phi Island at the start of the dry season, with long hot sunny days the prevailing feature.
Similarly the West Coast of Peninsula Malaysia is in the 'dry season', making Langkawi, Penang and Pangkor Islands great beach holiday destinations at this time of year.
December is high season for touring in Cambodia, Laos and Myanmar with low rainfall and slightly lower temperatures prevailing. The south of Vietnam including Nha Trang, Mui Ne and Phu Quoc Island should be ideal at this time of year.
Most of the Philippines is in high season and this is one of the best times of year to visit the Philippines.
In the Indian Ocean, Sri Lanka (with the exception of the east coast) is a great choice in December, as is the Maldives, which is a great Winter Sun destination.
Oman and the UAE are perfect at this time of year as temperatures are much more reasonable than in the summer months.