I don't generally go for the really weird stuff, but that's because I tend to like to go big with it when it happens...
I once ran an adventure for a party that started off fairly normally, with the party hired to protect a village from an evil wizard by a red-headed man wearing a strange amulet on his chest that let him cast
Antipathy/Sympathy several times a day.
But then it takes a turn into the strange...
When they get to the village, they discover that everyone in the village is wearing similar amulets that let them cast a particular spell. And everyone is a werebear. And the evil wizard wants to steal all of magic their amulets to build an infernal machine.
And then a turn to the comedic.
The party quickly realizes the adventure is full of '80's pop-culture references - the were-Care Bears are being threatened by a wizard who looks like the Purple Pieman from Strawberry Shortcake, and he needs their amulets to build a machine that will suck all the joy out of the world. To get to the wizard's castle, they have to travel through an enchanted Candyland forest, where the trees are made out of chocolate licorice with wintergreen leaves, with caramel
Squirrels running through the trees, following a blue-raspberry flavored stream. They encounter a friendly blueberry marshmallow ghost who guides them to the castle, where they fight gummi orcs and chocolate-covered-giant ants.
And then the adventure turns on a dime into the horrific.
The blueberry marshmallow ghost suddenly bleeds blueberry blood from its hollow eye sockets and lets out a banshee's wail. When the party fireballs the giant chocolate-covered ants, they melt into an ankle-deep pool of goop that smells of burnt chocolate, which the party must slog through to fight the gummi orcs. When a character hits the gummi orcs with a weapon, it becomes stuck in them with a horrific glorping sound and requires a strength roll to pull out. When one of the yellow gummi orcs loses an arm, a red gummi orc picks it up and attaches it to its own stump of an arm, where it quickly melds together into an orange scar...
And even worse, when the party makes its way to the wizard's tower, they discover he's actually a necromancer who's building a machine to suck the joy (positive energy) out of the world in order to turn the world's children into his personal mind-controlled army - whereupon the party is forced to fight a horde of
zombie children before taking on the wizard.
Yeah...
Afterward, the players told me that it was the greatest adventure I'd ever run for them, and that they'd kill me if I ever did it again, lol.