Riverside Park at 72nd Street, lower level: a 400-meter cinder running track with concrete bleachers, little changed from when it was first built in 1937. You can see the Hudson River through the trees, and beyond it, New Jersey.

References:

1. Interesting Places in the Parks of New York City, undated document in the October section of the 1937 Parks Department press release archive: “ WEST SIDE IMPROVEMENT - The plan for the West Side Improvement along Riverside Drive from 72nd Street to Dyckman Street includes a wide variety of facilities for active recreation .... A miniature Randall's Island track and field layout will be a feature.“ Like Randall's Island stadium, this is a track with bleachers.
2. NY City Department of Parks press release of May 26, 1939: “annual athletic meet for boys and girls of Park Department playgrounds in the Borough of Manhattan will take place at 74th Street and Riverside Drive ... which will include 18 track and field events...”
3. New Deal Assistance in NYC Parks Department Projects, 1934-43.