There's the obvious one: elves.
But is there anything different? Perhaps a trip to another plane (one with a manifest zone in the plane, like Mabar or Irian) could do the trick but then what kind of creatures could I use?
Anyway... what kind of creatures do you think that inhabit Aerenal?
Also, depending on the course your campaign is taking, you could throw in some good old fashioned negative energy undead like vampires or mummies, which represent rogue factions of the undying court too impatient or power hungry to wait for true undying status.
The rogue part of the undying court, however, doesn't really seem posible.
So using feywild creatures is my solution for the random monsters, but what can I use that is smarter?
I can't remember where they are described, but I think there is already an organization called the stillborn that are elves who use negative energy to become undead rather than deathless. Does anyone remember what I'm talking about?
ECS p.218
They are rebellious elves which are impatient with the all - "undeath must be earned" and are in league with the BoV to take over the island.
So you can confuse the PCs with an encounter where Stillborn undead elves fight some friendly deathless force.
See, I need a side trek for Zaenya (an elemental weird in Aerenal) to asing them in exchange for the information they are looking for. So I've decided that Zaenya can only give cold hard information or portents of hope.
But in order for her to give the party a cold hard fact, she needs them to bring her something she wants to know first (maybe what's in some magically sealed book that bad guys have) and in order for her to give them hope in some area, they must first bring hope to some person she wants.
I was thinking of using this part of the campaing to have the PCs discover Aerenal beyond the basic stuff (elves, undyings and forest).
But the good news is (for you) that I will be needing many posible side treks, because I'm sure they are gonna have a lot of questions.
So how does the society work? Presumably the number one goal for the living is to attain deathless status. In order to do that you must impress those on the undying court, who judge an elf or eladrin over the hundreds of years of their natural lives. I imagine that the entire society is rigidly traditional, so that new elves constantly try to emulate the way their society has been in millennia past. Unlike in normal adventuring society, individuals shun risk-taking and value doing exactly what is expected of them. Almost all decisions of importance are made by deathless, with the living falling all over themselves to prove how faithful they are to their deathless superiors.
As for the deathless themselves, they play subtle games over millennia. One of their primary occupations is actually maneuvering and deal-making to promote those living elves that they see as sympathetic to their own goals to being deathless. Once an ally is made deathless, you can reliably count on them for . . . eternity, so such gains are subtle but extremely important to them.
All of this combines to make elven society seem slow and utterly tradition bound to outsiders. However, the deathless are also wise and if a matter concerns them all they quickly set aside their personal politics and act decisively and with the combined experience of hundreds of thousands of years to solve it.
I know that doesn't particularly help you with your adventure ideas, but maybe it will color NPC attitudes as your PCs adventure.
How about an abounded Draleus Tairn armory where the weapons (and other objects) have been animated to life (due to the Irian manifest zone). The surrounding jungle is full of traps and hidden bunkers still maintained by a detachment of elite Dragonborn soldiers remnants of the Elf-Dragon Wars which have not aged since.
Ok, that is brilliant! Specially considering that one of my PCs is a dragonborn that happens to be in Aerenal investigating about the Dragon-Elf war because a seer (or perhaps just a crazy woman) told him that the side he took on the war that his ancestors have long battled will forever mark the fate of the world. So far he thinks this war she talked about is the Dragon-Elf war.
But what could I use for the animated armors and stuff? Hobgoblins?
hmm...
You can always make a legion of Baboons for them to fight if all else fails.
a legion of killer Baboons were bred by the ancient inhabitants of Zinj to guard the dragonshards mines from intruders...
You can also drop a Ruin Elemental in:
Great idea.