You can checkout the Washingtonian Magazine for happenings, a month in advance.
http://www.washingtonian.com/
Also, the Washington Times has a thursday "Weekend Mag", which gives the happenings. The Post has one on Fridays...
www.washtimes.com
www.washingtonpost.com
Explore the areas restaurants. There is quite a diverse of community, so there is a diversity fo food places. Lots of them aren't expensive, but have fab food. You can get anything from Russian caviar, to Bolivian Beef, southern-style pulled pork BBQ, to Starbucks coffee.
I heartily endorse the Smithsonians, and the art museums fascinated all five of my kids... My now 14-yr old daughter was captivated when they had the" Impressionists", and she fell for Dali, and Auguste Rodin's sculptures...
The Air and Space Musem has a Planetarium, and Imax, and shows worth seeing (even for us big kids)... Others have mentioned it, but you can spend a day in the natural History Museum. Orkin (the termite guy) donated a section that has LIVE WEIRD bugs, and they are Coooool! Tour their Websites, and make notes... You can miss a lot of stuff, and you will never see it all.....
http://www.mnh.si.edu/
If you want a little getaway, from DC, you may consider spending a night on The Appalachian Trail, at the Big Meadows Lodge, part of the Shenandoah National Park's "Skyline Drive". It was built during the 'Hoover Times", by the CCC, and offers a welcome respite from DC, with a view.
http://www.nps.gov/shen/
From there, it is an easy drive to Monticello, and visit the area that was home to many of our nations' founders...
A fun day with the kids could take you to Harpers Ferry, WV, for a day of tubing on the Shenandoah, or rafting elsewhere...and a chance to see 300 year old houses, and building, may built from trees that were made into 80 foot long rafts and floated down in the spring floods, then unloaded, disassembled, and made into buildings, and a town...
http://www.rivertrail.com/
http://wvweb.com/cities/harpers_ferry/
Most of all, plan on walking...and walking...and walking...
...then you get to go home, and deliver the film to the processors!
Good Luck, and hope you have as much fun as we do!
the A&S Museum downtown will keep you busy all day, and keep you downtown, where all of the good stuff is...
Crime is everywhere, not just DC. Don't run around scared, but be cautious. I drove to AZ last week, and on the way stopped at a rest stop to relieve myself. I was out of the car for maybe 3 minutes, but had left the drivers door unlocked. OOPS, I lost my camera bag, with my Canon EOS Elan, with 3 lenses, my new, Canon Digital Rebel camera, (which uses those lenses, also), my, almost-new JVC digital camcorder, and a great pair of Nikon binoculars. The $$$ weren't as much a loss to me as the exposed fim, and the 4 videotapes in there, with my Grandson' birth, and 1st birthdays, and my granddaughters' Christmas. (if anyone sees my stuff, with a black nylon bagf, I offer a $1000 reward)
Keep aware!