"Freedom is the right of all sentient beings."
- – Optimus Prime
Arborea (known in pre-Planescape times as Olympus, with both names later being combined into The Olympian Glades of Arborea) is the Chaotic Good plane in the D&D cosmology and is a pretty sweet place to be.
It's a realm of freedom and plenty, you can get away with pretty much anything here and indulge in endless decadence for all eternity. But unlike some other mystical hedonists that we know of, because the plane and its inhabitants are Good aligned, the experience is more about teaching you something positive, rather than ensnaring you in a trap meant to leech your soul until it rots.
You could spend your days enraptured in endless orgies and shared intravenous drug use, but you'd probably get bored and rather than become desperate to find a more extreme high, you'll likely be encouraged to turn your pursuits to something else more constructive. If you were an evil dickhead who just wants to cut on innocent maidens with a chainsaw, then you're in the wrong bloody plane and will be dealt with swiftly - go to the Abyss if that's the sort of fetish that you're into, and beware, because other, stronger people might do similar things to you (and that's what you deserve).
This is the place where it makes YOU a better person, rather than many of the lawful good planes which exist to make society better.
Like Arcadia, the plane is a realm of plenty and ease, forests are rich with fruit and fields are always filled with grain, the seas are filled with fish ready to be caught. But unlike Arcadia there is no regimented order to anything, and you might have to work a bit to find what you seek. The same goes for trying to find those mythical orgies, since this is also a chaotic plane no-one here exists to serve your every need, and they could just as easily tell you to get lost, but politely, since it's still a good plane.
Arvandor is the terrestrial layer, with the appearance of untamed nature. You've got forests, mountains, lakes, rivers and pretty much anything you can think of stretching into infinite directions.
The Elven Court live here, along with Wenta, and it's probably what the plane is most famous for, although the Olympian pantheon (Zeus and such), plus Rhea and Lirr (though she technically lives in a city within Olympus), make their homes here too. The third big place besides those two is Brightwater, where the several Forgotten Realms goddesses: Lliira, Sharess, Sune, and Tymora live along with Ye'Cind.
The Gilded Hall of the Sensates
An infinite ocean, but unlike the Elemental plane of Water it has a bottom and a top, so it is far more hospitable. Though of course you'll need a boat. There are no islands here at all, and everything goes on beneath the waves, so unless you can breathe water you'll find there's not much to do other than sail around all day catching fish. Oh, and watch out for the storms.
Beneath the waves, since Arborea is still the plane of celebration and plenty, all you need to do is find the lyrics to Disney's "Under the Sea" and you'll know what they've got. Deep Sashelas hangs here in his realm of Elavandor alongside Trishina, when not with the rest of the Seldarine. Poseidon also has his own realm, Caletto, in here.
Aquallor is the end point of the River Oceanus, which is like the good version of the River Styx, albeit considerably shorter since it only flows through the chaotic side of the upper planes. So if you get bored of endlessly sailing the infinite ocean, you could take the proverbial river upstream and back to the Beastlands or into Elysium, though how you'll find a "river" in an ocean of water is beyond me.
Contrary to the rest of the plane, Mithardir is actually not very hospitable. The layer is an endless desert of white dust. Stories go that once-upon-a-time it used to be a forest inhabited by Titans and Giant races, but they've all gone elsewhere. Though most gods and pantheons have also abandoned it, Nephythys, guardian of the dead, remains here.
You can still find dust-worn ruins in the desert, and there might be hidden secrets of a primordial era lost and forgotten here, but unless you know what you're looking for there is really nothing else here and you could end up getting lost in sand-storms and dying of thirst.
Some of the most interesting things about Arborea are the inhabitants; interestingly, you've got multiple types of petitioner rather than souls arriving and taking a common form.
Many Chaotic Good people who die and turn up here become Celestial or Anarchic animals, fairly standard shit. If you worshipped the Elven pantheon, you get to come back in Elven form, whether or not you were an Elf before you died. That way, you get to spend your afterlife enjoying yourself with other humanoids.
If you were someone who simply relished the "act of living" (i.e. a hedonist), then you become a Bacchae, which is like a Saytr but is its own thing (since being a petitioner is a template rather than a creature type); you spend your afterlife in a constant party of drinking, dancing, singing and shagging. They can become a bit of a roving menace because they have this mystical quality of enticing non-bacchae into joining their parties, though the new members don't get any sustenance out of the food that Bacchae can conjure, so they eventually collapse out of starvation or thirst, unless you somehow don't need to eat or drink, in which case you are pretty much trapped. Thankfully, the Bacchae will probably have moved on by the time you wake up, unless you've died from the experience, or if someone manages to save you by shooing the party-satyrs away, because they're typically non-violent and will just up-root and party somewhere else.
Beyond the petitioners, there will be mortals of a variety of Chaotic-Good races who choose to spend their lives here, because it's still a pretty good place to be.
The Court of Stars
One very strong mention needs to go towards the group of good celestials that live on the plane: the Eladrin, which are like Elf-Angels. They have their own hierarchy and their own leaders too. Queen Morwel is the leader who oversees all Eladrin affairs, and she delegates politics between her consort Faerinaal and her champion Gwynharwyf, and it is strongly suggested that she engages in bisexual threesomes with them. Their headquarters is called the Court of Stars and it is a demiplane within Arborea that can only be reached with the Queen's consent, and she can open portals anywhere in Arborea she chooses, even within the divine realms of the gods.