Vietnam would be a great option if you plan and book your flights in advance. Lots of different landscapes and things to do- you could pick north, south or central depending on the duration of your trip and your preferences.
Best and cheap option would be Thailand.
no visa fee, cheaper airfare and but for December make sure to have advance booking since it is festive season.
If solo backpacker, book dormitories you get in cheaper rates than fancy hotels or Vietnam
You can even come with us @Women Voyagers, we are into Ladies, Women, Moms travelling in groups. We are going to Bali from 23rd Sept to 28th Sept 2019. you can enjoy Bali with your friends. Alternatively you can Like and Follow our Facebook Page @Women Voyagers, and get to know of our latest trips and travels to exotic locations. Budget ka budget and ofcourse enjoyment with your friends..