Below is one of the more interesting historical postcards I have found. Not because of the postcard, obviously. It was a blank slate on both sides.
Before Charles Lindbergh flew his trans-atlantic non-stop solo flight, he was an airmail carrier, and flew the St. Louis - Chicago route. He did this for a year, and then left to build the Spirit of St. Louis aircraft. After his famous flight the US Post Office convinced him to fly the airmail one more time as a promotional event. He made flights on February 20 and 21, 1928. Each piece of mail received the horseshoe stamp seen below, which reads "Lindbergh Again Flies the Air Mail." There was so much mail, multiple airplanes had to be used, which meant Lindbergh couldn't fly them all. The Post Office Department assured everyone that Lindbergh at least took each plane for a trip around the airfield, so each piece of mail got flown by him, at least for a short distance.
1) "Lindbergh Flies His Old Mail Route," New York Times, Feb 21, 1928, pg 13. (Proquest Historical Newspapers)
3) Charles Lindbergh Discussion Center