Superman #66 (July 1950) featured the story “The Last Days of Superman!”. Superman has been getting ill and weak lately. At first thinking it is a piece of kryptonite, he is later alarmed to find out the piece of kryptonite has been removed and concludes he must be dying from over-exposure to kryptonite.
What results is an interesting take on what he would do with his last days and how we would help the Earth for years to come, including finding vast amounts of coal, oil, and setting up solar power plants:
He later learns that scientists predict that a planet will crash into Earth in 1987, so he zooms out in space to remedy that:
After doing all those good deeds (uh, why didn’t you do those in your first couple years as a superhero?), he resigns himself to his fate and leaves a message to Earth on the Moon, signing it “Superman (Clark Kent)”. Of course at the last minute, he realizes that a splinter of kryptonite fell into a photographer’s camera case and that is triggering his episodes. After removing the splinter and editing the goodbye message, we are back to the status quo.
The premise of the story is cool – what would Superman do in his last days if he knew he was going to die. It has been revisited a couple times in later years. Once in 1962 (Superman #156, “The Last Days of Superman!“) and once in 2005 (All-Star Superman, which I have yet to read).