Hanging out between my math and programming classes I added as much level design as I could. (And I still have an hour to spare).
I focussed on adding a sense of scale to the world. I wanted the world to seem TALL because you will be traveling from the depths of the deepest cave to above the highest clouds.
Between the sky and caves there is somewhat of a neutral area (the fields). Here there are no obstacles or hazards; only catchy tunes and some pretty blades of grass.
This neutral area is now four screens high from the entrance of the sky levels to the entrace of the cave levels. From there, both the sky and caves are about 4-6 screens high as well.
I also designed it so that you can take a leap of faith from the sky all the way down to caves with ease.