1st Idea: We can have a zone where we can build atrium or indoor gardens for our passengers to get a better variety of the airport especially for nature lovers. I think we don’t need to get very far with it but leave that to hiring landscapers and let them work on it. The higher the skills, the better is the design as well as better maintenance.
2nd Idea: Research/ Prestige Points/ Reputation allow us to unlock several attractions or facilities. At Changi Airport in Singapore they have a butterfly garden, lily garden, pond for rare fishes or a massive aquarium, cinema. As we unlock them they are build-ready, all we have to do is place them on the areas we want to make the place more lively for the passengers. Maybe we can have some requirements like how much space needed or how many passengers have to visit the airport in a period.