The Palm just south of Dupont Circle on 19th street has a decent priced lunch special. It is 3 courses for $20.

A little north of dupont in Cleveland park is Nam Viet. It has decent Vietnamese food. Not as good as in VA, but not bad at all.

If you like traditional Korean fare, I can't recommend Mandu. If you want sojutinis and cheap (but low quality) mandu, then give it a shot for happy hour. It is best when the drinks and apps are cheap and you can sit outside.