Gary Gygax devoted a fair amount of attention and detail to the Lower Planes of AD&D, in both the rulebooks and especially his Gord novels, but left the Upper Planes mostly untouched except for names and a few supernatural creatures that dwell there (ki-rin, titans, baku, hollyphants, devas, etc.). His reason for doing so was presumably twofold - firstly that such realms don't offer the same sort of adventuring potential as the lower planes (putting Good deities to the sword and looting their stuff doesn't have quite the same appeal of doing the same to hordes of demons and devils), and secondly that as someone who was strongly religious in his personal life Gary was always very careful about "treading too close" to subjects and content that impinged upon real-world Judeo-Christian religion.
Nevertheless, even if there's not so much adventuring potential on these planes there is value in detailing them from a worldbuilding perspective, to give players a sense of what those realms are like (and what their characters can expect after death), as well as to provide a foundation to build upon in the event travel to one or more of these planes (intentionally or otherwise) does come up in play.
In that spirit, I've collected both notes from the AD&D canon of what is known about each of the Upper Planes (including which creatures and deities dwell in each of them) and have added some color and flavor of my own based on what felt fitting and appropriate for each plane. These are rough notes from a couple days of brainstorming activity and not intended to be complete or final, only a starting point and outpouring of ideas and inspirational nuggets to guide and inform potential future development.
Note that any resemblance between the below and other post-Gygax depictions of these planes (including TSR's Manual of the Planes and Planescape) is wholly coincidental and comes only from being derived from the same base (Gygax-era) source material.
Weather is calm towards the lawful planes, turbulent towards the chaotic planes; climate is universally mild (warm or cool depending on specific plane/level) - it never rains on the lawful plains (but doesn’t create drought conditions), it rains (or snows) daily on the chaotic planes (but doesn’t create floods); it is never full night on the upper planes - daylight fades (in scintillating reds and oranges) to perpetual post-twilight with deep indigo sky and brilliant starlight; on the neutral planes there are always puffy white clouds in the western sky and menacing storm clouds in the eastern sky.
All senses (vision, hearing, smell/taste, and even feel) are sharpened and amplified on the upper planes, but at the same time visitors always have a sense of detachment and unreality similar to a waking dream or hallucination - it all feels “too real” to be real.
Food and drink are plentiful on all upper planes (type depending on the plane) and require minimal effort in cultivation and preparation; eating a meal of upper planes food will instantly refresh the eater, functioning as a combined elixir of health, potion of vitality, and potion of extra healing (effective once per day per individual), however, for every such meal consumed the eater has a cumulative 10% chance per meal of completely and permanently forgetting their mortal life. Any food or drink taken away from its native plane loses all extraordinary powers and becomes normal food in all respects.
Architectural style, clothing, cuisine, types of flora and fauna, and all other visual trappings are variable based on the culture and expectations of the visitor - everything will always appear comfortable and familiar but may well appear differently to a visitor (or deceased soul) from a different culture or alternate prime plane. The deities who dwell in and rule over each plane are able to modify such appearances at will, as often as desired.
The primary and most common inhabitants of all the upper planes are mortal souls of humans and demi-humans (save elves, whose spirits dwell temporarily in Alfheim before being sent back to the mortal realm). The lawful planes are densely populated towns, the neutral planes less so (small villages of at most a couple hundred souls), and the chaotic planes are sparsely populated (single-family homesteads). In their natural state these souls have no memory of their mortal existence (or more than a few days in the past) and are wholly content to repeat endlessly their narrow pattern of post-mortal existence (though certain spells, such as Speak With Dead, will temporarily bring back their mortal memories). They do not fight (except for those who feed themselves by hunting) or use any magic. They can all communicate with each other and any other person or animal via telepathy. They cannot be permanently killed by physical means (they regenerate 1 hp/turn from all forms of damage and will always be fully restored within 24 hours) though some spells - such as Destruction - may do so. They will never willingly leave their plane/level unless extraordinary magic is employed. If attacked all animals in the area (which are actually immortal spirits in animal form) will come to their defense. If that is not sufficient, the more powerful supernatural beings native to that plane will intercede and the offenders will be detained, ejected, or slain depending on the magnitude of their offense. The majority of post-mortal souls dwell on the first level of each plane - the mass of regular, mundane individuals of the corresponding alignment. The upper levels of the planes are inhabited by the souls of the most pious and exemplary, the heroes and saints of the various religions.
Inhabitants of the Upper Planes generally: Agathion, Deva, Hollyphant, Planetar, Solar
[See also Heroic Legendarium p. 89 for guidelines regarding how magic items and spells are affected on the Outer Planes]
Endless Town (cobbled and lighted streets and open squares with inviting cafes and taverns and shops, comfortable bourgeois homes (warm hearths, soft beds, plush sitting chairs), no garbage or foul smells, no rats or pestilence, no crime)
- Rustic suburbs (gardeners (vegetables, herbs, small livestock) - St. Cuthbert, The Protector)
- Midtown (crafters and artisans - Pholtus, dwarf gods)
- High street (bankers, jewelers, luxury goods - Marduk)
Deities: St. Cuthbert (Greyhawk), Pholtus (Greyhawk), Marduk (Babylonian), Moradin (Dwarf), Berronar (Dwarf), Clanggedin Silverbeard (Dwarf), The Protector (my pantheon)
Endless Leisure (1st level is an endless beach with warm water and gentle tides, higher levels are ivory buildings and amphitheaters on puffy white clouds)
- Sport (Girru)
- Games (chess, etc) (Heironeous, The Father)
- Intellectual pursuit (philosophy, debate) (Chung Kuel, ki-rin)
- Sculpture & painting (Ebisu)
- Dance (Kuan Yin, platinum dragon)
- Music (Surya)
- Poetry (Vishnu)
Deities: Heironeous (Greyhawk), Girru (Babylonian), Chung Kuel (Chinese), Kuan Yin (Chinese), Surya (Indian), Vishnu (Indian), Ebisu (Japanese), The Father (my pantheon)
Endless Savannah (wild cattle, horses, and dogs, antelope, impalas, gazelles, zebras, giraffes, elephants, buffalo, rhinos, etc.)
- Lowlands (Epimetheus, Halfling gods, The Mother)
- Uplands (Ukko, Gnome gods, air maidens)
Deities: Ukko (Finnish), Epimetheus (Greek), Garl Glittergold (Gnome), Baervan Wildwanderer (Gnome), Segojan Earthcrawler (Gnome), Flandal Steelskin (Gnome), Yondalla (Halfling), Arvoreen (Halfling), Cyrollalee (Halfling), The Mother (my pantheon)
Endless River (self-maintaining croplands on either bank - wheat, barley, millet, sorghum, lentils, peas, beans, dates, figs, watermelons, pomegranates, coriander, cumin, coffee, sugar cane, rice)
- Delta (Egyptian gods, The Grower, phoenix)
- Lower river (Ishtar, baku)
- Upper river (Sumerian gods)
- Headwaters (other gods, moon dog)
Deities: Ishtar (Babylonian), Isis (Egyptian), Seker (Egyptian), Ushas (Indian), Tsukiyomi (Japanese), Bragi (Norse), Enlil (Sumerian), Nanna-Sin (Sumerian), Nin-Hursag (Sumerian), The Grower (my pantheon)
Happy Hunting Grounds
Endless Forest (elk, deer, moose, squirrels, birds; fruit, nuts, berries, mushrooms)
- Boreal (American gods, Brother Sun)
- Temperate (Skerrit)
- Jungle (Bast)
Deities: Raven (American), Hotoru (American), Snake Man (American), Bast (Egyptian), Skerrit (Centaur), Brother Sun (my pantheon)
Endless Mountain (storms)
- Forested foothills (Alfheim - elf gods)
- Slopes & vales (pastoral (sheep, goats, alpaca, rabbits) - Nephthys, foo creatures)
- Cloud-capped heights (godsland - Greek gods, titans, eudaimones)
Deities: Nephthys (Egyptian), Zeus (Greek), Aphrodite (Greek), Apollo (Greek), Ares (Greek), Artemis (Greek), Athena (Greek), Demeter (Greek), Dionysus (Greek), Hera (Greek), Hermes (Greek), Nike (Greek), Poseidon (Greek), Prometheus (Greek), Tyche (Greek), Corellon Larethian (Elf), Deep Sashelas (Elf), Rillifane Rallathil (Elf), Aerdrie Faenya (Elf), Erevan Ilsere (Elf), Solonor Thelandria (Elf), Hanali Celanil (Elf), Labelas Enoreth (Elf)
- Midgard (realm of endless cliffs, fjords, volcanoes; storm-tossed sea - Tritherion, Indian, Japanese gods)
- Asgard (realm above (mountain-tops) - Norse gods, The Son, valkyries)
- Niflheim (realm below (caves) - includes Muspelheim, Nidavelir, Svartalfheim)
Deities: Tritherion (Greyhawk), Karttikeya (Indian), Lakshmi (Indian), Hachiman (Japanese), Oh Kuni Nushi (Japanese), Odin (Norse), Aegir (Norse), Balder (Norse), Forseti (Norse), Frey (Norse), Freya (Norse), Frigga (Norse), Heimdall (Norse), Idun (Norse), Loki (Norse), Magni (Norse), Modi (Norse), Sif (Norse), Thor (Norse), Tyr (Norse), Uller (Norse), Vidar (Norse), The Son (my pantheon)