Lovely pictures! Meridian Hill Park (first two) is a gorgeous spot. One of the few Italian Renaissance parks in DC.
Fall is the best time of year in Washington, but it's over so quickly.
When I moved to Washington in 1967, the leaves began changing around Columbus Day weekend and were usually gone by Veteran's Day. I remember my first Thanksgiving, there was actually a fairly significant snowstorm Thanksgiving afternoon and into Friday.
All of DC's neighborhoods are great in the fall, but my favorites were Georgetown, DuPont Circle, and Capitol Hill.