The beauty of the mildly-neutral Outlands is that it accomodates encounters from ANY planes....it's a crossroads, where creatures come to be on a common ground. This is also interesting because it can include outcasts -- rogue modrons living in isolation, deserters from the blood war avoiding conscription, errant celestials fed up with taking orders from some jerk deity. The Outlands is so vast, it's easy to hide in the wilderness there.
It's also probably the plane closest to the elemental nature of the Material plane. It's neutral. Natural things like plants, animals, elementals, and fey are going to love it. Dragons, probably, too. It also has critters familiar to the Prime underdark, like mind flayers, and beholders.
Some sample encounters:
Deserters A Marilith and her Nalfanshee armada have given the slip to the Blood War, and have decided to set up camp on some innocent independant Outlands town. The Marilirth is calling the shots, and the Nalfanshee don't like it -- they WANT to go kill devils. But since she's the most powerful thing for miles around, they ascquiesce...until the PC's get there, anyway....
Dare I? A Forlorn (from Bastion Press's MINIONS) is seen holding a hapless merchant at swordpoint. The Forlorn is caught in a quandry. Obsessed with balancing good and evil, this merchant (who cheated a town on his items to help feed the daughter he is raising by himself) presents the creature with a question he can't answer: should he execute him for his crimes, or allow him to live? The PC's are roped into helping the Forlorn solve this problem, and the merchant is terrified the entire time. Both success and failure can lead to a caravan's worth of treasure from the poor merchant.
God sighting! The PC's spot a great deity walking towards the Spire. He is surrounded by the smell of brimstone, and varrangoins (from the Fiend Folio, a troupe of which may attack the PC's). Mostly this is just eye-candy to ensure that they know this isn't a normal plane.
Bariaur! The PC's stumble on a camp of bariaur engaged in their games, and the friendly goat-folk challenge the PC's to a contest of strength. What the PC's don't nessecarily know is that the leaders of this tribe are retired planewalkers who have achieved stupendous power (they're average level 17). The contest of strength could turn against the PC's, but either way they'll probably be rewarded for participating with something from the planewalker's hoarde.
Rilmani are wierd A troupe of ferrumach rilmani (from the Fiend Folio) suddenly emerge from the bushes with a look of utmost urgency on their faces. They plead that the PC's stay and listen to them, for the sake of the entire multiverse. One of them draws X's in the mud of the path, and another asks the PC's to please, please stand on them...and a few seconds later, a great roar peirces the air and a spurt of hot blood washes over the party -- miraculously missing them if they stood on the X's. Investigation reveals that a hunter stealthily slew a sleeping thunderbeast (from the Tome of Horrors) next to the path, and the blood is from that creature. Why the rilmani didn't want the PC's bloody is never really explained, as they quickly hurry off the path into the thick underbrush.
We police our own A celestial seeks assistance to capture and kill an errant celestial who is hiding out here. A Justicator (from MM3) has fled his responsibilities, and the celestial wants the PC's to help kill it. It knows things that cannot be allowed in a deserter. Of course, the creature is still good, and lawfully good, but has fallen from the true path...do the PC's help? Or do they turn against the paladin-Justicator who has emplored them to help?
