In addition to the major sites such as the National Mall, C&O Canal, and Rock Creek Park, and Fort Dupont Park, the National Park Service manages many of the city’s triangles, circles, and squares.
How to tell? Look for brown signs like these or visit
https://www.nps.gov/locations/dc/parks.htm to find one near you.
Fort Stevens sign, 2011, photo by MrTinDC via Flickr, Creative Commons License