Der Ballon schwebte am Himmel.

English Translation

The balloon was floating in the sky.

I would want to say “im HImmel”?

“Himmel” is ambiguous, meaning both “sky” and “heaven”. So the preposition is kind of used to disambiguate:
am Himmel - in the sky
im Himmel - in heaven

Imagine the german “Himmel” as a ceiling that things are attached to. So things are “at the ceiling” instead of “in the ceiling”.

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