What did the returned exiles rebuild first, before the temple's foundation was laid?

Answer: The altar of God

In the seventh month they set up the altar and offered burnt offerings, although the temple's foundation had not yet been laid.

“Then Jeshua son of Jozadak and his fellow priests, along with Zerubbabel son of Shealtiel and his associates, began to build the altar of the God of Israel to sacrifice burnt offerings on it, as it is written in the Law of Moses the man of God.”— Ezra 3:2 (BSB)