Imagine a world in which clouds are composed of round transparent membranes pasted together, living bubbles, that float in the air.
Here's how it would work: In a story universe I'm working on (Colony Zero) I've contemplated what it would take for an animal to breathe in carbon dioxide and exhale oxygen. Clearly there would need to be an input of energy from the outside, since this chemical reaction requires more energy to process than it releases. But plants do this all the time with photosynthesis. What if there was a creature who likewise had a chemical reaction requiring outside energy input (like photosynthesis) that split water into hydrogen and oxygen?
Returning to the little round bubble body concept I mentioned, imagine the creature I'm describing filled it's inner sack with the hydrogen it produces. (It probably would also need to absorb CO2 and convert it to oxygen to get building material for its body, not to mention energy, but that's a bit beside my point here.) So each one of these life forms would be like a floating jellyfish. Clumped together in a huge colony, these creatures would be nearly indistinguishable from a natural cloud. I'm imagining them about as intelligent as a jellyfish as well, though someone could make each one communicate with each other in a such a way that a single cloud could function as a collective intelligence.
The interesting thing about these clouds is in theory someone could walk on them, depending on how tough the body membranes of the cloud creatures are. If they are very weak, you'd fall through the cloud, but it would slow your fall. If a bit stronger, you'd sink into them like quicksand. Stronger still and you'd be able to walk on them like walking on a jump castle or a trampoline. A bit firmer still and they'd give you a bit of a bounce, but you could walk normally.
Especially if they were relatively firm, a human being could build a literal castle or other structures on one of these clouds. Or other creatures, especially flying ones like griffins or dragons, could nest up there. (Note that a careful planner could build on a relatively soft cloud by spreading a wide foundational platform that avoids breaking bubbles.)
Please note that being filled with hydrogen would make these clouds highly flammable (the Hindenburg, anyone?). In fact someone might say that these clouds would be impossible due to the build up of electric charges between ground and air that naturally occurs. One lightning bolt between the colony and the ground and it's game over for the living cloud creatures. However, this could be mitigated by making the membranes of the creatures naturally flame resistant. Or they could somehow fill themselves with hot air instead of hydrogen (I think that's a bit harder for a simple animal to maintain, but it could work for story purposes).
Or perhaps these cloud creatures could maintain long tentacles that trail down to the ground and drag them along as the cloud moves across the sky, serving as a ground (or "earth" if you're in the UK) to equalize the charges, preventing a deadly lightning bolt from striking in the first place. This would allow for some interesting possibilities, such as the cloud getting anchored to a tree by accident or a strong human or other being having the chance to climb up to the top of the cloud using one of the tentacle ropes. In fact, the problems of tentacles would probably imply a creature with enough intelligence to be able to raise or lower the tentacles, a single living creature floating on hydrogen instead of a colony.
As a final note on this story idea, while it is true what I'm presenting is original to me, I know that some similar ideas have been proposed. Especially in the atmospheres of gas giant planets it's been suggested that floating creatures could exist (I recall Carl Sagan saying so back in the 80s). I also know that I've heard of floating animals in a fantasy context. I haven't heard of floating animals that look just like a cloud in other stories, but I would not be surprised at all if another writer has used this idea before.
But as you can see, this story setting concept has quite a number of different potential applications. I doubt all of them have been used. I don't as of yet have a story of my own using this idea, but I may get to it someday. May other writers and story lovers find the possibilities as interesting as I do!