Theodore Roosevelt
Noted for his advocacy of the "vigorous life," Theodore Roosevelt did not retire quietly. He traveled to Africa from 1909 to 1910 with naturalists and taxidermists from the Smithsonian, where they acquired hundreds of plant and animal specimens for the Institution's collections. After that, Roosevelt toured Europe, and returned to the United States as popular as ever.
Unhappy with the administration of his successor, William Howard Taft, Roosevelt eventually formed an alternative Progressive Party. He did not win the national election of 1912, but remained visible, writing books, traveling to Brazil, editing magazines, and campaigning for the Republican presidential nominee in 1916.