Depending on what you want to spend I would recommend the Carribean. Florida can be beautiful at that time but I have seen the reverse and had cold winds in late December. The Carribean is in the tropics so you can be guaranteed some warmth. You might check out how much the fares are to Hawaii - again it will be warm but it is their rainy season. I have had trips where te weather has been perfect - I have had others where a Kona low has meant rain for days on end. If bdget obliges you to stay a little closer to home I would recommend Palm Springs for warm days but chilly nights.

Any of the above will be warmer than St Louis I think.