Old Town Alexandria (Virginia) is beautiful to walk around at Christmas. My wife and I go there for our anniversary occassionally (12/23). This hotel is three blocks from the King Street Metro. Great restaurants all down King Street.

Several hotels there, but the Residence Inn at the hyperlink seems to be open all week and is recommended. The main/fun part of King Street is a long walk from the hotel, but there is a free trolley stop a block away that takes you down the whole street. Shuttle will as well. There is another RI a couple blocks away, nearly as good of an option, just more offices around (but I have gotten a 2 BR upgrade at that one).

Easy Amtrak service to King Street...I wouldn't bother driving. Maybe six blocks total Amtrak to RI, or $3 taxi.

I'd be VERY surprised if you don't love this two night trip.