In the biblical Book of Genesis, the first thing Noah built after leaving the Ark was an altar.
Genesis 8:20 says Noah built an altar to the Lord after the Flood, once he, his family, and the animals had left the vessel. He then took some clean animals and clean birds and offered burnt offerings on it. The altar is therefore the first construction explicitly recorded after Noah’s departure from the ark.
The story continues with God’s covenant and the rainbow as its sign. Only later does Genesis 9 describe Noah planting a vineyard. This sequence explains why “vineyard” is a plausible but incorrect answer: it is an important later event, but not the first building activity named after the Flood.
The verse also connects the earlier animal distinction to Noah’s response after the waters recede. Genesis had instructed him to preserve extra clean animals and birds, and those animals now provide the offerings described at the altar. The account does not say Noah first built a permanent home or city.