The Morrison Clark Inn at 10th and Mass NW has a Sunday Brunch. Beautiful patio and porches. Lovely dining room located in an historic Hotel on the National Register. Local and seasonal foods. They've just gotten a new chef and I haven't eaten there since she's taken over but I've heard good things. Have never had less than a fine meal at this place.
There are even some tables out on the porch if the weather is good.