What were people doing in the days of Noah before the flood?

In the days before the flood, people were eating, drinking, marrying, and giving in marriage. They were engaged in normal business and living—working and building—while the earth was also corrupt and filled with violence. They continued until the day Noah entered the ark, wrapped up in their world and heading for their doom.

Les teaches that the days before the flood were marked by ordinary activities: eating, drinking, marrying, and giving in marriage. He explains that people were working and building, doing all the activity of normal business and living. He notes there is nothing wrong with those things in themselves, but they were under such intense activity, and along with the corruption and murder, they were heading for their doom. Les points out that even after the ark was finished, God left the gangplank down and the door open for seven days. Those wrapped up in their world could have stepped in, escaped the flood, escaped God's wrath, and enjoyed salvation, but none took it except Noah's family. Les applies this to today, saying the world is similarly busy.

Matthew 24:37

But as the days of Noe were, so shall also the coming of the Son of man be.

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Were the people in Noah's day doing something wrong by carrying on normal activities like eating, drinking, working, and marrying?

No, the normal activities themselves were not wrong. The text notes they were eating, drinking, marrying, working, and building, and there is nothing wrong with those things. Their problem was being consumed by those activities while corruption, murder, and approaching doom surrounded them, and the people did not enter the open door of the ark.