My idea comes from my own game that I last created and played.
The characters are given a challenge by whatever means you want in your game setting.
The challenge is:
Travel to an ancient and wealthy city renowned for its thievery and loose morals.
Once there, you must find the ONLY honest man in the entire city.
When you find this man, you must scam him out of 100 gold pieces (this should cause any Good characters to have an uproar and argue with the Neutral or Evil ones).
Let the characters do some local info gathering or role play it however you want, for the characters finding who the honest man is.
The "Only Honest Man" turns out to be the greatest fence (stolen goods re-seller) and a top ranking official of the powerful and evil Thieves Guild!
The only reason he is honest, that his entire reputation is on the line as a fence, and nobody would bring him their stolen goods if he was not reliably honest.
And he may just have a quest of his own for the characters, since he's not amused by their weak attempt to scam him. Or perhaps he'll send an assassin to hunt them down for him?