Thanks guys! I tried to use translucency for the balloons, but it didn't look right, so I used a little luminosity, maybe a little too much. I have to say that I used a fill light setup for this image. I tend to use this most of the time, because when I decide to animate the scene, there will be no GI flicker.
For the clouds I use the same cloud layer twice: one with normal settings and the other one with the same density shader, but with much lower density but a little more coverage to get some fuzzy area around the cloud bodies (I hope that makes sense!?)
I added the houses right before I hit render. The .tgo is a single set of some houses, so it might look weird if you'd see, that some of them are buried into the ground and some of them are floating (cheater!!!)
OK, I'm testing the clouds a little more, because this was my initial intention. Thanks a lot for your tips anyway!!