Historical Sites:
Arlington National Cemetery - the Changing of the Guard at the Tomb of the Unknowns is VERY moving. The row upon row of perfectly place headstones. Also, President Kennedy's grave site.
Gettysburg - Looking across the open ground from the Union position to where the Confederates came out of the woods is sobering. Trying to imagine 15,000 men breaking from those woods in an attempt to take the Union position really makes you think on the discipline of those Johnny Rebs. Also, the fact that only half made it back when the attack failed.
Dachau - experienced this hellish place on a cold and damp morning. From the time I arrived until I left a feeling of sorrow was nearly overpowering.
Concord Bridge - where the "shot heard round the world" was fired.
Liberty Hall - Where the Declaration of Independence was signed
Antietam - Standing on Burnside's Bridge and looking over the battlefield I could only imagine the carnage of the bloodiest day in the Civil War.
Ford's Theater - Where Abraham Lincoln became the last casuality of the Civil War.
West Point - As a kid I was with a tour group and actually got to eat lunch in the dining hall with the cadets. All that history.
Rome. Florence, Venice, London - too many locations of historical significance to mention.