Museums and Experiences
The Smithsonian Museums are a must-see while in DC. Among my personal favorites is the National Arboretum (hardly any tourists go there because it's off the beaten path) and the National Zoo.
Here are some links to non-Smithsonian Museums and Experiences that are just as great:
World Class Shopping
When people think of DC they usually think of museums, monuments and politics but the DC area also has world class shopping experiences.