Make sure you purchase an "all day pass" for the Metro in DC. It takes you, almost everywhere you would want to go in DC, practically drops you off at each tourist/history destination.

Also, as you are probably aware, almost all of those destinations are free to get into/explore, etc.

My personal favorites, were the Vietnam Wall, something extremely special about that place.

And, the Smithsonian Museums.

To be honest, I'm not a "history buff", so I was quite surprised at how much I enjoyed DC. I could easily spend three days there, but one day is also very good.

Enjoy!!!