I really like the floating cities idea. It'd probably be perfect for a habitat on a habitable exoplanet. No need to worry about local flora and fauna coming along and infecting or poisoning the inhabitants, since it's built in the middle of the ocean with terrestrial plants and animals. Also, I wouldn't be surprised if there were some of those 2km pyramid-arcologies on Mars; it's the sort of thing you'd build there until terraforming got far enough along you could survive outside in the right morph.