I highly highly recommend doing Steakhouse 55 at the DLH!! It is awesome, and it's nice and dim, quiet and can be romantic or casual, whatever you make of it!! The food was superb!!
I also like Carnation Cafe, but for me that's more like a quick lunch thing that's yummy, much better than all counter service!
I also love Storyteller's Cafe, and Wine Country Trattoria in DCA!!!
Andrea