One of Washington's least known monuments is the National Japanese American Memorial near Union Station. It includes a water feature in zen garden style and a magnificent monument to all those who struggle against injustice, in the form of two cranes trying to free themselves from barbed wire (photo from http://www.njamf.com).
Closest Metro is Union Station (Red line). The memorial itself is at D Street & New Jersey Avenue, NW,
Washington, DC 20001.