Have you been to the National Arboretum yet? It’s honestly one of my favorite areas in DC. Covering 412 acres it even has this amazing group of columns that are originally from the United States Capitol! It’s a great place to explore with lots of trails with a variety of gardens.
With it’s blend of nature and classical design, it really is a beautiful backdrop setting for a photo session! I recommend doing a photo session here during either the early morning or near sunset to capture the golden light hours.