My bf and I went last year 12/9-12/12, and it was great...really low crowds. You should also try to check out MVMCP...there's dates on 12/6 and 12/8 this year, it's a blast and totally puts you in the holiday mood.
As far as romantic resorts go...I don't have any firsthand knowledge of staying there, but I love the atmosphere at the Poly. It's so fun to lay on the beach at night in the hammocks there...we even caught someone's wedding fireworks from the GF!