Arlington...........
I second Arlington, especially if your kids are old enough to comprehend what it represents. The changing of the guard at the Tomb of the Unknowns is worth seeing. I don't get to DC much, but I feel that a visit to Arlington, although memorable and also somewhat enjoyable in the spring, is somewhat of a duty - its the very least we can do as citizens, to recognize and humbly honor the sacrifice of those buried there by visiting their graves.
Just thinking about Arlington is one of the few things that cause me to hold back tears.
Also, I seem to remember some news report about the cherry trees blossoming prematurely this year because of a warm spell. Don't know if that will adversely affect the usual show.