In a ring surrounding the inner planes lie the Elemental Planes. They form a circle, with water and fire on opposite ends, and earth and air also opposite each other. They are technically a single plane, but are usually divided into four distinct zones. The elemental planes has played host to a power struggle between the jotnar and genies for millenia.
Sometimes, the innate magic of the elemental planes lashes out through the Bifrost and forms a portal to the Inner Planes. This can lead to an incursion of elementals, giants, or dragons. Sometimes these portals can become permanent in locations of elemental power, such as a volcano or waterfall.
On the Elemental Planes, distance and direction does not have the same meaning and rigidity as they do in the Material Plane. All these planes have a defined center, and the further you go from there, the wilder the elemental forces becomes, until you eventually reach a different elemental plane. These liminal spaces are strange and dangerous, but contain powerful magic. For example, at the border between the Plane of Fire and the Plane of Air are the Howling Dunes, a land of scorching hot desert winds.
The Nexus
At the point where all four elements meet is called the Nexus. This is where the primal energy is the strongest. Very few mortals have travelled to the Nexus and returned. Some believe the Nexus contains the ultimate secrets of primal magic, others that it is a prison that holds a roiling chaos that must not be let loose.
Skyfarheim, the Plane of Air
The center of the Plane of Air is a collection of islands, floating on clouds in an endless sky, far above the plane of water. The largest island, Aetheria, lies at the center of the plane, and is the seat of Emperor Yan-C-Bin, who rules over a myriad of princes and nobles, all vying for power in an intricate game of favors and counter-favors. All beings who make this plane their home must know their place in society, or risk offending those with power.
A giant thunderstorm roils around the Plane of Air. It is the source of many dangerous lightning elementals and other dangers, who sometimes comes close to the floating islands near the center. If you fall off the islands, if you are not caught in the magical winds that howl throughout the plane, you eventually end up in either the great ocean of the Elemental Plane of Water after falling through the freezing reaches of Frostfell, where air and water meet, or you fall into the black sands of the Fire Plane, after passing a great firestorm at the border of the planes.
Politics
The emperor Yan-C-Bin is not as powerful as one might expect, as most of his political clout is tied up in making sure he stays emperor. If the elector-princes are unhappy, they could replace him. Yan-C-Bin sends his servants to find magical artifacts which he gifts to electors in order to keep them in line. If a pretender showed up with richer gifts, why would the electors keep the current emperor around?
Portals to Skyfarheim
All thunderstorms carries some of the power of Air, and a powerful one probably contains a rift across the Bifrost to the Plane of Air. Adventurers from the Material Plane had better come prepared though, as where they end up probably does not have solid ground.
Vatnaheim, The Plane of Water
The Elemental Plane of Water is an endless ocean. There are settlements here which lie on the shallower parts of the seabed, or cling on to small atoll islands. Coral reefs, kelp forests, and deep ravines hold many dangers. Chief among them is Jormungandr, the World Serpent, who will come to Midgard during Ragnarok and devour all life in the oceans.
City of Lantea
The center of the Plane of Water is a rocky island, upon which a very crowded settlement is built. The city is built in concentric tiers, separated by large canals. Outside Lantea proper, the settlement continues underwater for many miles until it peters out.
Buildings in Lantea are made from blueish stone, and often have domes and spires. The innermost tier is the Azurerock, the great palace of King Oceanus and his court of marid nobles.
It is said among sailors that the city can sometimes be seen in the distance from becalmed ships on the Material Plane.
Portals to the Plane of Water
The Plane of Water maintains a connection to the oceans on the Material Plane, and is probably the easiest elemental plane to travel to - if you can survive the crushing pressures and lack of air at the bottom of the ocean.
Myrkheim, the Plane of Earth
The Elemental Plane of Earth consists of a great cavern system, sprinkled with swamps, bogs, and small settlements. It is the least civilized of the elemental planes. Further from the central chambers lies unexplored tunnels, filled with gems and riches but also dangers.
Portals to the Plane of Earth
Some portals are located deep underground, through large geodes of amethyst, quartz, or topaz. Others are on the surface, where muddy whirlpools of mud act as one-way doorways to the Plane of Earth.
Muspelheim, the Plane of Fire
The Elemental Plane of Fire, or Muspelheim, is a hot desert of black sand dunes, lava rivers, and volcanoes. The warmongering leaders of the fire plane raid other planes for slaves and glory.
City of Igneum
The city at the center of the plane. Ruled by Surtur, the Pharaoh of Fire, a primordial Fire Giant who commands armies of giants and efreeti. Azer slaves are plentiful, as are slaves and servants of other conquered races.
A river of lava flows through the city, providing heat to furnaces and other industries. Fire giants make up the ruling class, bolstered by efreeti soldiers and slave-drivers.
Palace of the Pharaoh
The great pyramid palace is covered in shining black stones. The surrounding complex has many buildings decorated with arches and domes. Several thin spires poke at the dark sky, but none higher than the peak of the palace itself.
Surtur receives audiences in a large throne chamber at the center of the pyramid, lit by flowing lava. He stands over twenty feet tall and wears a robe of rich crimson. On his completely bald head is an open helmet of obsidian, set with bars of ruby. This headdress is the foremost crown jewel of his royal house. He wields a crook sceptre, wrapped with black leather.
Odin has foreseen that Surtur will invade Midgard at the time of Ragnarok, when the planes will intersect and the gods will do battle on the mortal plane.
Portals to the Plane of Fire
A volcanoes or hot spring might contain a permanent portal to the fire plane. A temporary portal could be either the result of, or cause of a forest fire.