Volo's Student's Guide to the
Great Wheel
The Great Wheel is the grand Planescape where many of the most prominent Planewalkers do much of their planewalking. It consists of a dozen and a quarter plus one Outer Planes (detailed here), many of which contain multiple layers. It also contains several Inner Planes, as well as the Material Plane of Greyhawk (and used to contain some others, like the Forgotten Realms as well, though they've since reconceptualized their cosmology... metaphysica, feh!).
This brief overview of the Outer Planes of the Great Wheel cosmology is dedicated mostly to what might interest students on weekend retreats or summer vacation.
The Abyss
For reasons no one can adequately explain, adventurers love this place. It's violent, hazardous, and your chance of surviving here is next to nil. Actually, that probably explains it.
Notable Inhabitants of the Abyss
Bill (glabrezu): A glab' Mora knows.
Bob (vrock): A friend of Mora's from the Abyss.
Fred (glabrezu): A glab' Mora knows.
Kardugan the Terrible (glabrezu): The toll attendant at the Vivisection Pass toll booth, and would-be terrible demon BBEG. To be truthful, he said his name was Kardugan the Terri... (choked off), so he may be Kardugan the Terrific, or Kardugan the Terriculous, or Kardugan the Terrine, or maybe just Kardugan the Terri.
Lenny (vrock): Full name: Lannarlongdividalliandilar. A friend of Mora's, from the Abyss.
Relkinth the Defiler (balor): Writer of tanar'ri romance novels. (name created by Biklik)
[Thief of Your Candy and Burner of Your Eyes] (glaberzu): A glab' Mora knows.
Acheron
This is that plane made of floating cubes you've heard so much about. Travel between cubes is expensive, so bring lots of gold. Visitors are likely to become paralyzed, preserved inanimates. Avoid the local restaurants; this is one time where the chains are the way to go.
Arborea
Very peaceful, very relaxing. The spas are well-worth every penny.
Arcadia
They think they're absolutely perfect, which is one of the best reasons to avoid this place. Snobs.
Beastlands
The hunting is excellent here, if you're in to hunting, but make sure your license is up to date. Ehlonna does not kid around with the quotas, so don't get greedy. You may end up being the next hart shot.
Bytopia
No, the layers are not in danger of falling on top of each other. You will not be crushed. Also, the natives have heard every "bi-" joke you can think of, so just keep it to yourself, chuckles.
Carceri
Seriously. Has anyone ever been here? Ever?
Celestia
One big, nigh infinite mountain, which you'd think was good for mountain climbing vacations, but really, unless you're searching for salvation, it's just a lot of climbing.
(see also: Tinya)
Elysium
The source of the River Oceanus, and one of the most kick-ass river cruises of all time.
Gehenna
The biggest problem here is heat. And gravity on a 45 degree slant. Bring suntan lotion. And pitons.
Hades
Hades is the bleakest, boringest plane in the Planes. I mean, sure, the Blood War is waging here all the time, but even they're bored. There's a theme park, but it doesn't have rides. Just an ice show.
Hades locations
Hades Harry's Used Carts: A used cart dealership, where the prices are craaazy. But dull.
Hades Harry (petitioner): The lethargic and dull "craaaazy" salesman at Hades Harry's Used Carts.
Limbo
Limbo is raw chaos. Seriously. Wild magic tables aren't enough. Rods of Wonder? Feh. You'd need a table that includes "Penguins tap dance by, and explode on the completion of the triple time step" and "your sword turns into a pickle, but keeps its magical properties".
Notable Limbo Inhabitants
(see also: Cado Lost Mountain)
Mechanus
There are waaaaay too many laws here, and you'll be arrested for violating any of them. And you will; even if you have a complete legal code, you're bound to miss something. Like the law making it illegal to read the legal code in school zones. If your lucky, the Modron March will come by and distract the arresting officer.
Notable Inhabitants of Mechanus
Fate (kolyarut): An agent of universal law, who seeks out those who do not contribute to the continuity, and erasing them from existence. Somehow, he's managed to convince Primus to help him out in this regard, though the One and the Prime is pretty much over it by now. Fate was destroyed by Korwan Truesteel under advisement of the Inevitable council of Mechanus.
Omicron-11 (Formian Warrior): Member of Mechanus's police force.
Trivac (zelekhut): Inevitable designed to serve human servants of law, as a sidekick. He is programmed with a sense of humor and the ability to make judgment errors, to make humans more at ease. He's one of those weirdo robots that start all their sentences with the type of sentence he's saying. ("Statement: I am here to assist", "Interrogative: What the hells, man?", "Interjection: Help!")
Nine Hells (Baator)
Ruled by devils, Baator is a tourist's delight. Everything you could possibly want can be found here. For a price. Which is usually your soul. Dispater especially, is probably up to something. Seriously, though, you'd be surprised how social those dark princes are.
Notable Nine Hells inhabitants
Zbarva the Witch-Queen (human cleric-5/Sorcerer-9/Disciple of Mammon-5): One of Mammon's minions. She's just using him for the spell-like abilities. Whether he's aware of it doesn't matter. He's desperate for the minions. (based on the Book of Vile Darkness character; property of WotC)
Outlands
Look, way up there. That donut thing? That's Sigil. Wave to your friends. They can't see you; they're inside, and it's an infinite distance up. Your magic doesn't work here. While you're looking up and waving, and lacking any sort of abjurations, someone has lifted your wallet. Welcome to the Outlands.
Pandemonium
It's windy. Wear a coat.
Ysgard
Adventuring students come here by the droves, for the chance to fight epic battles to the death, without worrying overmuch, since they resurrect via the plane's own energies within twenty-four hours. Adventuring students are very, very stupid.
The Elemental Planes
The Elemental Planes of Air, Earth, Fire, and Water have little in the way of tourist industry. However, they are popular spots for skydiving (Air), scuba diving (water), spelunking (earth), and burning alive (fire).
Notable Elemental Plane inhabitants
Jenny the Janni (janni): A janni opportunist, willing to sleep with any genie who can further her position. She really hates Ombaran.
Krazazahd (noble efreet): Powerful, formerly alcoholic (until Amizath wished he was no longer an alcoholic) genie who recently suffered a string of bad luck. He has an attraction to illithids, and presently dates/serves the devil princess Fierna.
Organizations of the Inner Planes
Arrowhawks: An air genasi street gang in the Elemental Plane of Air, who enjoy choreographed dance knife-fights.
The Energy Planes
I'd say "stay away", but that'll just make a bunch of reckless college kids want to come. So, let's just leave it at that.
(The Great Wheel, and all planes listed here, are property of Wizards of the Coast)