If you wanted to go to Florida, I recently stayed at Acqualina and really enjoyed it. I don't know if they have minimums over Christmas, but one place I am interested in trying is the Sunset Key Guest Cottages in Key West. But - that is probably too far for your timeline.
What about Cabo? Originally, you said North America and all suggestions have been in the US so far. Cabo would be really nice and warm in December and easy to get to for you.
Austin, TX is a fantastic place to visit. It could get cold. I have family in the area and they are often in tshirts while I am bundled up in snow clothes.