S.,
I lived in D.C. the year I had my first son. I was not working so I had a TON of time to see all the sites. I LOVED IT!
Will you be traveling with kids? If so, what are their ages?
There is so much to see. Some things that I particularly enjoyed were the Library of Congress, the Renwick Gallery (an Art's and Craft Museum). Of course all the Smithsonian's are amazing. The newer Native American Indian museum is great. You can really taylor your visit to meet your interests since all the museums are amazing. Art, history, science - you pick.
Some of the most popular sites have timed entries. Arrive there in the morning, get your entrance ticket (usually free) and then come back during your time slot to get in. It saves on waiting in line. I think that the Archives can be like that on busy days, and when I lived there some of the busier museums had that as well.
There are also great neighborhoods to walk if you are interested in the history or architecture. Georgetown is super fun, and has an interesting historic canal (which I believe you can take a ride down - I never did). There are also boat tours that leave from Georgetown and take you down the river.
I lived near Dupont Circle, which is where all the embassy's are, which are interesting as well.
I also agree, that a walking tour of the monuments at night is great! I walked down there regularly with my husband and we always felt very safe after dark. Especially if you are right around the main ones between the WWII and Lincoln Memorial.
The Smithsonian Zoo is also fun for kids - they have elephants, pandas, and other unique animals you don't often see at smaller zoos.
Send me a message if you want more info.
If you have specific questions, let me know. I could probably answer or make recommendations.
** One more thing - D.C. can be REALLY HOT and HUMID in the summer. Keep the climate in mind when planning your days. Plan to see museums during the hottest parts of the day. If you think you want to spend time walking in Georgetown, I wouldn't go during the afternoon, rather plan it for the a.m. or for evening - it's a great place to get dinner.
Jessica