I found this great panorama of the 1900 Paris Exposition, so I added a lot more sky, expanded the parchment border, and started rendering the flying gardens. I used Strata 3D to model the gardens, the taxis and most of the people. I discovered it was easier to just illustrate the people, and splice them in via Photoshop later.
A happy accident: when looking at maps of the 1900 Paris Exposition, I discovered that the horticulture section is directly below in this view. How appropriate!
By the way, it's worth your time to Google the Exposition. Amazing level of technology for 1900. They had electric moving sidewalks in parts of it. There some are images here.
Source for city view