Harmon had already taken the lead in developing several properties in the New York metropolitan area, notably Pelhamwood starting around 1909 and continuing to develop Harmon-on-Hudson (now part of Croton-on-Hudson), initially developed in 1903 as an artists colony. He also continued with real-estate development in other cities such as Philadelphia, such as the development of Penfield in Haverford, and Louisville, Kentucky. The Clifford B. Harmon & Co. continued to advertise until around 1930, after which the firm presumably closed.

Early aviation and other pursuits Harmon seated in a Farman III biplaneHarmon was known for his love of automobile racing, yachting, horses, and other pursuits. In 1908 he learned to fly in approximately one week, and was awarded the sixth pilot's license issued in the United States. He was involved with both ballooning and airplanes; in 1908 he set a record for time aloft in the Indianapolis balloon race. In 1909 Harmon participated in several balloon races; he again participated in the June Indianapolis balloon race, with A. Holland Forbes; his balloon was fired upon at night traveling over Morgan County, Alabama. In September 1909 he was the subject of a large search when he flew solo over western Massachusetts and lost contact with those on the ground; Harmon landed safely and returned to city via car and train.

In October 1909 at the St. Louis Centennial balloon race Harmon again set a flight duration record of 48 hours 26 minutes as well as an altitude record of 24,000 feet in his gas balloon New York. Later in October he was a witness for a Wilbur Wright flight of 27 minutes (the longest at a government aerodrome at the time).