The setting is an endless sky of naught but pillowy white clouds and floating islands scattered across the skyscape. In such a world, how would the various nations, from small city-states to large empires, trade with, conquer, and otherwise move people and resources between one another?
The islands sizes vary from small acre-large landmasses to landmasses the size of Australia (with the average landmass being somewhere between the size of Liechtenstein and The Netherlands). Some landmasses are closer to one another, and some further apart from one another. There are some groups of landmasses which are somewhat close together, with large distances between these groups and others. These are called Archipelagos (but can be thought of as possessing local diversity and complex international relationships in the local sphere much like a continent).
The average distances between islands in the world is quite large. Even "close" islands are out of visual range for all but the clearest days with optimal light conditions, where they appear as small specks in the sky. "far" islands are tremendously far away and cannot be seen at all. Of course, there exist some islands which are significantly closer to one another such that even birds migrate between them, but most fall between the two bounds mentioned above.
There is no correlation between any of the 6 cardinal directions and any effects on temperature, pressure, or other environmental features. There are indeed some cold and wintry islands, as well as hot barren desert islands strewn across the sky, but the climate in these areas is unrelated to their position relative to their neighbours.
Note: There exist strong "currents" similar to tradewinds in various places in the sky (due to concentrated flow of mana / Ley lines / dragon veins). These currents are rather localised and can be though of as behaving similar to a slipstream, speeding up travel along their course. They don't cover the entirety of the sky; but split and merge in a complex network which doesn't double-back on itself. The currents are dynamic, but take up to hundreds of years to split, merge, or move by any great degree.
Viable Aerial Creatures
There are no man-made airships, but there are a variety of tameable wyvern sub-species (which can effectively be grouped as "big" and "small"), as well as sky whales. Wyverns are bipedal reptilian creatures possessing webbed wings. The big wyverns can be thought of as being capable of carrying up to ~3 people, and the small wyverns only a single passenger (the pilot). Sky whales are gargantuan creatures, many times larger than an actual whale and up to hundreds of metres* long. They are peaceful creatures, flying from island to island, but are out of the control of humans.
Big wyverns are capable of making the journey between close islands at maximum capacity with ease, and they can make the journey between far islands with fair difficulty and at reduced load. Small wyverns can just about be able to make it to close islands, with far islands out of the question. Sky whales are able to fly indefinitely with ease.
Note: If it helps, there are technically dragons as well, capable of carrying many tens of people, however they are noble and intelligent creatures of incredible power, and would only ever offer their assistance if particularly enamoured / interested by a human or situation. Unlikely to volunteer for goods delivery.
With the exception of invading close smaller countries, which may be possible with a collection of large and small wyverns, and given the restrictions provided, I feel that sky whales offer the greatest potential as a large-scale logistics solution. The primary concerns that remain unresolved being that the sky whales are meant to be the paragon of freedom, and having them work with / subordinate to the terrestrial races would compromise this image.
Sky whales will often fly by islands and loiter about before being on their way.
Pros: Sky whales retain their "freedom" and means they can have goods loaded onto them (somehow).
Cons: For the sky whales to be loaded up with cargo implies knowing where they're headed next. With a clear migration pattern, they feel less whimsical and free-flowing. Unless docked, loading and unloading cargo would be a complex operation perhaps requiring smaller aerial creatures.
Once near an island, sky whales are benefited by some form of hospitality such as cleaning, prompting them to dock.
Pros: Similar to above, but more conducive to large-scale logistics operations.
Cons: Feels either like the sky whales are being used, or otherwise like a quid-pro-quo arrangement. Both of which detract from the imagery with which they are to be associated.
I'm hoping for some form of solution that allows for the fairly large-scale logistics that would be necessary to facilitate complex trade / military events; either by modifying the above ideas or recommending new ones. I'm happy to clarify or expand on any of the above points.
* I'm bad at approximating sizes. Maybe larger, maybe smaller. Gargantuan nonetheless.