There's a flower library on the Mall, over by the Tidal Basin, near the Washington Monument. Every spring it bursts forth with tulips shortly after the cherry blossom bloom.
This year the library bloomed spectacularly, as usual. My sometimes-noontime running group ran by it earlier this month.
My running buddy likes to seek out the different flower amidst a group of similar flowers. She calls them outliers.
There's always something to go see on the Mall. DC is a great running town.