Check closely the visiting requirements on the website. Because of heightened security you will need to schedule your visit. But it's worth the small extra trouble. And the kids should love it!
Everyone is giving you great ideas for educational things to do. I agree, definitely walk the memorials. You can start at Lincoln and then do Vietnam and Korean right there. The WWII memorial is fantastic.
But on the uneducational side - There are some lovely restaurants along the water in Georgetown. If the weather is good, it is nice to sit out at them.
Oh, and the National Zoo. I hear the new panda is adorable!
If you have a car and can get out to Great Falls (either Maryland side or Virginia side), the Potomac River is beautiful there. On the MD side you can climb on the rocks and even down to the river at points. Or just walk along the canal. The VA side has a nice path to walk as well.
It is also fairly inexpensive to do a Capitol River Cruise along the Potomac. You get a great view of the city that way.
All of the above are great suggestions. Arlington a must. Once there, do not cross back over the bridge without going to the Marine Corps Memorial (Iwo Jima).
Also, I would highly suggest driving from Arlington to the Capitol (or vice versa) at night. Absolutely beautiful.
Anyway, have fun, stop by Walter Reed on friday night and say hello to the D. C. Chapter. They're great people.
Air and Space Museum
White House Tour
Smithsonian
I always wanted to tour the Bureau of Engraving and Printing, but I was always there on the weekend and they were closed. I have no idea if it's a worthwhile visit or not, but it always looked cool.
The entire Smithsonian Museum.
When I was stationed in that area I made it up at least 4 or 5 days a month for a year, and still didn't take it all in.
In addition to everyone else's suggestions, you could spend part of a day in Annapolis, MD. The town is pretty, and you can visit the US Naval Academy there. Old Town Alexandria and Georgetown are good for shops, restaurants, and looking at lovely old houses and gardens.
http://www.usna.edu/visit.htm
I am having the same argument at my house. My kids want to go to Florida and I want to return to DC!
Last summer I went to DC for a week and one of the highlights was FReeping in front of Walter Reed! Your kids are old enough to participate and I would strongly suggest you do that. The DC FReepers are wonderful Patriots and will help you with directions etc. It will be a memorable experience for your entire family. Check out some of their threads...they are out there supporting our troops EVERY Friday night, no matter what the weather!
My teenage son's favorite was the Air and Space Museum. I loved the American History Museum!
As others have suggested, contact your Congressman and Senators now. Going through your representatives you get a "free" tour of the Capitol, and you see much more of the "behind the scenes," than if you do the public tour.
In one day you can see the Vietnam Memorial, the WWII Memorial, the Korean Memorial and this statue of Einstein isn't too far away and fun to see in person!
I can't wait to go back! Have fun!
Thanks for the ping, USF!
Buy a multi-trip Metro Pass and park your car and ride the Metro.