You could spend a week in Washington DC and not see everything in the Smithsonian's. We had originally planned on spending a few hours in one after walking the memorials and the few hours the next morning in the others, but with the bad weather we did them back to back, which meant we burnt out early (it had been a long day) and missed a key exhibit at one of the museums (more on that later).
We finished at The Natural History Museum, where we totally skipped the top level and hence missed the Hope Diamond. Other than the Hope Diamond this museum covers early human origins, the development of world cultures and mammals, birds, amphibians, reptiles, insects and sea creatures. While we were there they had a photography exhibit on that had some spectacular shots.