Good spot for birthday cocktails/light fare
This is a tall order:
--great atmosphere (think The Gibson or Marvin, without wait/crowds)
--can accommodate 10-15 people for drinks and light fare on a Saturday night
--in NW DC
--good for 30& newly 40-something :)
How about the lounge at PS7? Call them ahead of time and see if you can reserve a portion of the lounge area (I've seen them do this before).
Don't do Marvin, my friend had her party here and it was terrible. Rude staff, weak drinks. Terrible night.
When you say light, do you mean healthy? small plates? appetizers/bar food but main focus is drinks?