When Phineas and Ferb reach the Moon with their skyscraper, you can see the U.S. flag that the astronauts planted on the far left side of the screen and an abandoned lunar rover on the far right. Both are upside down, of course, as they reached the surface of the Moon.
Doofenshmirtz: Oh, I get it. I think I get it. Th-the light from the moon is just a reflection; from the light of the sun? Oh, Dunkoff! I should have rotated the sun!
Song: Floor After Floor.