One of each per city, and only once the city is big enough to deserve it.
Landmarks are prestige projects. Each lifts land value across a wide area and nudges the whole city's happiness up, and each draws tourists who spend money. They are never profitable on their own (their visitors always spend less than the upkeep), so build them when the budget is healthy and the city needs the lift in desirability or happiness, not as an income source.
A four-tile landmark museum of the city's art and history. Lifts land value up to 8 tiles away (+12 at the gates) and city happiness by 2 (all attractions together add at most 9). One per city; needs 6,000 residents. Built by a crew over about 40 days; it opens when done.
Visitors spend up to $60/day, always less than its upkeep · +22 jobs
A four-tile palm house and gardens. Lifts land value up to 7 tiles away (+16 at the gates) and city happiness by 2 (all attractions together add at most 9). Its grounds clean the air (−4 pollution). One per city; needs 12,000 residents. Built by a crew over about 45 days; it opens when done.
Visitors spend up to $80/day, always less than its upkeep · +14 jobs
A four-tile stadium for the city's teams and concerts. Lifts land value up to 10 tiles away (+10 at the gates) and city happiness by 3 (all attractions together add at most 9). Match-day crowds are loud right beside it. One per city; needs 20,000 residents. Built by a crew over about 60 days; it opens when done.
Visitors spend up to $200/day, always less than its upkeep · +40 jobs
Figures are the game's base prices. Upkeep is charged every in-game day, and construction bills are drawn from the city's stock of materials, steel, copper wire and timber (see farms and trade).