Unstacked cities pose one of the greatest challenges to Civ VI players. Sometimes bonus yields from carefully placed unique improvements can be more significant than bonuses from district adjacency.

And when land is sparse (like on water maps) this is even more critical.

A secret for those pursuing a culture victory: any improvement that yields culture will provide tourism in the late game. Yes, not just The Great Wall!

Spam kurgans around pastures for optimal faith output.

The Colossal Head can be built by the suzerain of La Venta, the city state. Build it surrounded by forest and/or rainforest for maximum faith.

Clear out features (rainforest, forest & marsh) and harvest bonus resources on flatland along rivers in order to place Ziggurats for culture bonus.

Very easy to place optimally, the Kampung should be built nestled in the middle of as many fishing boats as possible, for the most food.

Build sphinxes around wonders for the most faith. (Try the achievement Walk Like An Egyptian).

Build your campus in the open and get science bonuses from your missions. If you build them on foreign continents you get bonus faith.
(Note: the Winter 2016 Patch increases faith output)

Place holysites in the open to maximise faith from stepwells. Build farms between them for bonus food. (FYI not displayed is the +3 housing the stepwells provide the city).
(Note: the Winter 2016 Patch increases faith output)

Begin with improving your pastures, cos as Australia it will "culture bomb" adjacent tiles on which you build your Outback Station. Unlike most other improvements, the Outback Station is a contigious multi-tile improvement. Clear out flat grasslands and plains adjacent to your Outback Station to improve and expand its borders.

Place the Nubian Pyramid optimally adjacent to the city center, ideally on floodplains or else on hills. Then build your districts around them to boost their yields.
(Note: the tile yields show production bonus from the pantheon Lady of the Reeds)

Chateau are harder to place optimally. They're best next to wonders and luxuries for the most culture and gold. Remove features and harvest resources next to rivers to place them.
(Note: the Winter 2016 Patch increases culture yields)

The hardest of all and requiring the most decision making is The Great Wall. Get bonus to gold, culture and tourism from building a long continuous wall rather than a short segment.