The Mall at night
You can use the Metro and do the Mall at night. Best to alight from Metro at the Capitol South Station. Start from the Capitol building and work your way west. For taking photos, I recommend to bring a tripot. Don't miss the WW II Memorial, its fantastic at night!
When you are done, you probably have to back track to the Smithsonian Station and take the Metro to Crystal City. Be sure to check time for last train.