We did this last December and it was fabulous. The beautiful US Botanic Garden gets all decorated for the holidays AND puts up an incredible working model train display in a side room. They are doing this every Tuesday and Thursday evening so I strongly encourage you to visit (on this night or others). It is one of the nicest holiday venues in the area. Here's their website. (http://www.usbg.gov/) It's free, Metro is close, parking is easy, and the place is incredibly decorated and under-visited.
On this night, we'll enjoy the choral performers Capital Accord Chorus - so get your pipes warmed up! I don't think they sell any food or drinks, but you can bring something in. We will walk across the Mall to Teaism afterwards for a bite to eat. That's a really nice walk to make this time of year; we'll see the Capital Christmas Tree and other holiday highlights.