I have no idea whatsoever if you can use any of this stuff. A whole lot of it is really just random detritus I have floating about, both in my head and on paper. Here it is, maybe you can use it.
An evil mage by the name of Sinistratus (yes, that's his name, yes, I know) obsessed with immortality has discovered the means to store his vast and unfathomably evil intellect into a high-level spell. He has disguised this spell as some powerful 'oh-wow!' enchantment, scribed it into an obscure spellbook, and hid it in a demi-plane library where several scholars have been spending time studying a certain mystical phenomenon (possibly the plane of mirrors).
His idea was that someone would eventually find the spell, commit it to memory, and then the spell--actually being nothing more than a copy of his very 'essence'--would proceed to take over their mind and convert the hapless spell-caster into a copy of himself.
All in all, it was a pretty savvy idea--just one problem.
That 'obscure book' he chose turned out to be the all-important textbook of a powerful mage, and everyone on the team ended up memorizing the spell.
Sinistratus is long dead--killed by his own creation. All that remains of his legacy is the smoldering ruins of a once-prolific library, now constantly patrolled by dozens upon dozens of low-level mages who all believe themselves to be the original Sinistratus. All of them are completely, absolutely, irrevocably barmy--and attack anyone (including each other) on sight.
This wouldn't be so bad, except that recently a group of adventurers attacked the place, fought through a lot of the Sinistratus-clones, and managed to reclaim the book (which they assumed was a high-powered spell-book) for themselves--and their mage proceeded to memorize the spell, too. Things went downhill from there.
Now there are Sinistratuses spilling out all over the Plane of Mirrors. For most of the denizens, they're little more than a nuisance--but a few of them have been managing to escape, scribing the nasty spell rattling around in their heads to scrolls and trying to spread the plague (all the Sinistratuses are obsessed with what the original Sinistratus was obsessed with--attaining immortality--and they still all 'think' that this is a good way to pull it off).
The adventurers may encounter one or two lone Sinistratuses near the beginning of the adventure without much explanation, and may only later on stumble across the vast academy filled with Sinistratuses, either because a necessary part of their quest takes them there or just out of curiousity to find out what happened. The option to destroy the original copy of the book and spell should be available, as well as a potential NPC (one of the scholars who survived) who's studying a way to reverse the process using an engineered spell that targets anyone who currently has the Sinistratus-copying spell in memory.
The Planar Tourism Board's Official Mimir
"Congratulations on your recent acquisition of the Planar Tourism Board's official Mimir! Inside of my skull you will find an array of useful, helpful information to aid you in your decision on whether or not to commute from the Prime Material to the Planes..."
The Planar Tourism Board's official Mimir is the perfect gift for any player freshly arriving in the Planescape mythos--it's a century out of date and absolutely brimming with misinformation (what could be better?). The best part is that it was designed by a bunch of barmy Fated who had the crazy idea of attracting Primers to the Planes to increase business by painting everything in the Planes in a positive light.
Ask it about the Spire--"Gods got you down? Tired of mages throwing around polymorphs like they were hot potatoes? Travel to the exotic Spire, home of the rilmani! No annoying, errant spells here, no sirree!"
Ask it about Carceri--"Enjoy the heartfelt sincerity of the local color, all of whom desperately want to know just what they need to do to you to convince you to never leave!"
Ask it about Gehenna--"Visit their renowned hot-springs! Bring your swimming trunks--it's always summer here!"
Ask it about Baatezu--"ERROR, NO DATA ON RECORD"
I have more, but it's late and I'm exhausted--I'll write them up later and post them here. If you have any requests or something specific you want me to hone in on, go ahead and throw it at me.
Edit: These are both rather humor-related, but don't worry--not all my ideas are meant to be silly. Just MOST of them. D: