The pastiche Conan and the Treasure of Python has two chapters showing the 'public' area of Stygia's port [forget it's name] that outlanders are allowed to occupy under heavy guard. so they'd be allow to land then- even Stygia needs to trade w/ outlanders though it heavily regultes who gets the liscence to do so. Though Conan does infilitrate the more public areas he does so by swimin across crocidile/serpebt infested waters of the delta to the main city.
As for ideas-
The player while going through a more private section of the city, see what they swear to be a trusted ally from afar slipping into a small temple of Set or another foul god with several priests. Is the person really their once-trusted ally and if so wy are they consorting with the enemy? If not, is it a imposter to replaace their ally? Further investigation is required.
A Stygian adventurer intent on risking the swamps for black lotus hires the PCs as 'expendable' guards to hep him. Once they get to the groves of black lotus they discover the Stygian plans to sacrifice them to the spirits guarding the lotus for safe passage.
A Son of Set decides that one of them would make a lovely sacrifice. If they defeat it though, they have the committed a mortal sin in the Stygian 's eyes. Cue the angry mob.
Akashvia(sp?) or a lesser vampire of her get is hunting the city, preying on outlanders rather than her own blood. She/he prefers seduction to outright battle, but if hungrt enough will subdue and kill. The vampire may also be willing to bargain with the PCs to help her/him obtain another victim in exchanage for information/,material they only have.
Two Stygians get into a public sorcerous brawl. They are both killed and the PCs notice one dropping a object of gold and copper. The item is the key to finding an anceint treasure in the Stygian wilderness and was the object of the duel. The men's master lear of the PCs obtaining the key and send theri agent to rretrieve it. The tomb also has it's own guardians/traps. And what treasures lie in wait.
Raven, hoping it helps