This is back when gods could move planets. They would hire Superman to do it.
The pay was good, but the gods cribbed a lot. They made a lot of fuss about cosmic design, and each pantheon had its own ideas about layout and three-dimensional sculpture.
On his breaks, Superman would go surfing on solar storms or throw asteroids for Krypto to catch. In his free time, he also built himself a second Fortress of Solitude in the vicinity of Proxima Centauri.
After humanity died out and Superman was the last vestige of Earth, he moved there. The main hall of the Fortress was five planets wide. Sometimes Superman would play old movies on a wall, photographs taken by Jimmy, or home videos with Lois.
He had visitors – at times refugees – and they were welcome to stay as long as they liked. But they never lived long enough. He would go out and solve problems, he would bring back souvenirs, gifts from grateful cultures. He kept them all.
But most times, he would stare at the dying sun that Earth used to revolve around. Sometimes he would gaze into space and try to see faces in the stars.