Stop by Sibley Hospital. That's where I was born, believe it or not.
Yes, it's true. Buckeyemike is NOT a native-born Ohioan. I was born in DC, and my family lived in Northern VA.
In all seriousness, I highly recommend the Vietnam Memorial (maybe the most poignant place I've ever been) and the National Air & Space Museum and the American History Museum at the Smithsonian.
My parents actually met at the Washington Monument, so that's also a place I hold in high regard.