Making a Mark

You'll need this on the way to Key to the City - signature buildings are in the icon next to zoning. Each one has a different requirement for your city to be able to unlock it; the tooltip for each will explain what the requirement is; it'll be either X units of a certain zoning type, or & buildings levelled up to level 5 or similar. The first 5 (any 5) are pretty easy; pick the zoning ones.