and any other ideas about floating land masses I would be very grateful! ->I've loved floating castles/cities ever since seeing the cover of the old DND Dungeon Masters guide. There are many ways to answer this. Do you want to have the floating cities be a major part of the story & world? So for example, the cities constantly need to acquire resources. That might lean towards having cities land or dock at certain places to buy/trade or excavate the resources they need before going aloft again. This could provide a great way to bring characters to different parts of the world setting as well as travel to different world environments and peoples.
Permanent solution could be anti-gravity (tech) powerered by anti-matter reactors or air elementals bound into the object. An entire tribe or nation of magic casters could be enslaved that power the device/city to fly. The options are near endless. You could have a space structure from which the city is suspended. 922
The city could 'float' because it travels/flickers in and out of another dimension. Or maybe just pocket dimension bubbles that somehow got connected to this world, are floating around and somebody figured out that you can access them and actually live and build there.
Using something like anti-matter engines (kinda like the flux-capacitor in Back to the Future) you can generate enough power to do almost anything including powering enough low tech 'engines' to make just about anything fly. I mentioned anti-matter engine but really it could be any thing you want to make up. It could be the De-Gauss Nuclear Repulsor Generator(s).
Here's another idea. Mining the 'ice' rings around the planet for your 'magic' levitation mineral.
Or if it's a 1 off, it could be from some hi-tech war or technological experimental accident that caused some sort of chunk to float.
Maybe even flying mechs or creatures that rotate duty to hold the city aloft.
Check out movie Appleseed Alpha. It's huge walking mech. Could easily be transformed into a city. Ok, that should give you something to think about. 947