we did DC last year with then 11, and 9 year old. We did the engraving tour (make sure you get there early to pick up your tour tickets for the day before they are all gone. Go early, get tickets for later, then come back.
We also called our congressman's office and had a private tour of the Capitol! This was a highlight! You meet in a building across the street, and they take you over (actually under) in their little subway train, and you come up inside the capitol.
We also went early because we didn't know where it was, so early we figured we would eat something close by. No such luck, nothing around. However, they let us into the cafeteria for Govt workers in the basement, and it was the best /cheapest meal, and made to order! Breakfast for 4 of us cost about $12, and my husband got egss toast, hashbrowns, grits, coffee. I had a similar meal, as well as my daughter, and my son had lots of fruit and cereal!
Definately do Lincoln Memorial in the day, and at night. We also walked from the Capitol to the Lincoln Memorial one night. Very deceiving, we kept walking, and it seemed so close, but holy cow. We ended up taking a cab back!
We did all the museums, and the spy museum on our trip prior to this one. That one costs alot of money. I love that the museums are free.
We also ate at the food court in Union Station. Something for everyone.
Oh, and don't forget, the Nationals play baseball, and it is very close.