Casual Surf & Turf in DC.
Most of the steak places are Power Lunch/Dinner places, and full of suits.
For casual, probably best would be Kinkead's Mainly does fish, but does also do steak. As I have said before, I am not a Kinkead fan, but it is the best in this catagory. Casual is acceptable.
I would personally consider 1789 or Melrose. Both serve fish and steak, and to my mind, cook much better. I would think upscale casual would be fine in either.
If I were taking a guest to steak in DC, I would go to Les Halles for hanger steak and pommes frites. Shorts and flip-flops are acceptable attire.