If you don't want to use the Planescape model, I would suggest the following:
Air: Dark, storm-wracked sky, acrid black smoke instead of clouds, and barren stone islands drifting, shattered and scorched by constant lightning strikes, amid an endless void of thin air. Undead djinnis, Aarakocra, and elementals of smoke, void, and lightning wander the tortured skies.
Earth: Cavernous expanse of bleak darkness filled with narrow, twisting tunnels, muddy, flooded passageways, and countless dead ends, pitfalls, and other hazards around every turn. Few (if any) random encounters, but the PCs always feel like something is watching, just out of their vision range. Make sure to play up the subjective gravity and three-dimensionality of the place, and drive home the point that danger could literally come from any direction at any time.
Fire: Oppressive heat and choking smoke, but actual open flames are few and far between. Lava bubbles beneath the surface, cracking and rending the ground at random. Obsidian spines, abandoned towns of steel and volcanic stone crawling with burned undead and evil fire creatures. The horrors that lurk here are drawn to heat and flame, so the party will quickly learn to fear the light as well as the dark.
Water: A frozen wasteland wracked by constant blizzards and bone-chilling gales, the howling wind and blinding snow make sensing danger before it arrives all but impossible. Labyrinthine passages snake beneath the ice, providing respite for the cold and weary. However... the party will quickly discover that the desolation of the surface world is but a prelude to their troubles. They are not the only beings to seek refugee beneath the ice, and their neighbors are not eager to share.