I’ll be happy to try to help, but I’d really need to know your starting point because there are cheap places in different corners of the world, and it’s not worth flying halfway around the world to try to save US$20 per day. In mid December through January for beaches you should consider Thailand and perhaps also Vietnam. I’ll give you more information once I know where you are starting from. -Roger 

We are looking to travel somewhere warm in December of this year, for approximately 3-4 weeks. We are originating from Canada – Alberta, to be exact, and have done a fair bit of travelling throughout our lives, however, we are now bringing along our 4 year old son. With that in mind, I suppose our biggest concerns are now flight durations and safety (in our younger years and without children, these weren’t so much a factor, but definitely are considerations now).
My partner and I (in our 20s) are thinking about where to travel next December. We are from Australia, and since our last 2 trips have been to cold places (US and Japan) we are looking for a warm place to head to next. We aren’t into partying and I would love somewhere with nice beaches, wildlife and rainforests, but my partner can get easily bored so we need some activities and civilisation too. Probably looking at around 2-4 weeks depending on how much there is to see and do and would like to spend $100-200 AUD per night accommodation. Also, would like to feel safe wherever we end up.
If you’re a bit more on the timid side, you can always use Yelp and TripAdvisor to get good insights on places people have visited. As a disclaimer, some of the ratings may not be accurate on these sites, so make sure you go through and read some of the more recent reviews that people have written. We did this for a few of the places we visited and had no issues.