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Have the PCs go into business. They can earn one as a reward or just hire on as bodyguards. Then introduce the cutthroat competition. As in, they really want to cut your throat! Make it as mundane a business as possible, then introduce the most wild, fantastical D&D stuff imaginable.
"We are just trying to run a brewery here... What's a beholder want with our secret oatmeal stout recipe?"
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The Drunken Adventure (aka "Hey y'all watch this!")
If your PCs are the drinkin' type, they all end up very drunk. Then they start daring each other to do things to show how tough they are. Maybe a rich NPC gets the ball rolling by wagering gold or magic items.
"I bet you 100 gp you can't shoot that wizard's hat off with your crossbow! Har har har!"
An alternative to this one is that, while their decision-making powers are greatly diminished, a con-man offers to sell them a treasure map to loot hidden somewhere in the city. Maybe its just a diversion so the map-seller can get past the guardian. Or maybe he's testing the defenses in preparation for a major heist. Or maybe he's playing a joke on his friends.
At any rate, the PCs have to deal with the challenge while they are stone drunk. Attacks are at a penalty, spells misfire, somebody has to pee--REAL BAD.
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Something HUGE happens. The city is destroyed by earthquake and/or a volcano. All of the dead rise up from the grave and attack the living. A thousand demons gate in and attack. Two ancient dragons who hate each other choose the city as their final battleground.
What do the PCs do? What does the populace do? What about the city's rulers?
