Combined Suggestions, Washington D.C.
See as much as you can! Follow your own interests; there's something here for them. Whether it's history (Capitol, White House, Ford's Theatre), memorials (Vietnam Wall, Arlington Cemetery, Holocaust Museum), Space/Technology (Smithsonian), theatre (Kennedy Center), pop culture (Smithsonian again) -- whatever -- you can find it here.
Fondest memory: Touring the Capitol as Newt Gingrich walked past my shoulder -- while talking to Kurt Russell! A rather surreal moment.
Favorite thing: Be sure to see all of the sights. we sometimes will take a driving tour in a different direction each day only having to avoid rush hour.