After leaving Noah’s Ark, Noah first built an altar.
Genesis 8:20 says that Noah built an altar to the Lord after he, his family, and the animals left the ark. He then took some clean animals and clean birds and offered burnt offerings on the altar.
This scene marks the transition from survival during the Flood to worship and covenant renewal on the restored earth. It occurs before the covenant sign of the rainbow in Genesis 9, so the altar and the rainbow belong to adjacent but distinct episodes.
The answer is not “the ark” or a settlement: the story says Noah left the vessel and immediately built an altar. Later biblical passages use altars in many settings, but this is the first action specifically attributed to Noah after his departure.