OK, so you have this wonderful city setting, what are you going to actually do there? If you are short of prep time or are bedazzled by the possibilities and aren't quite sure where to start, here are four complete adventures plus 'interludes' that provide a framework for getting the party from first to fourth level. They appear in the Ptolus: City by the Spire sourcebook, and as one in a series of PDFs that present the same material. The adventures are all interlinked and play out on the streets of Ptolus so it's worth picking up the other source material for the city before running them. The adventures involve gangsters, politics and intrigue; and are embedded in the setting. By the end of them the party ought to have met some of the movers and shakers of the city and will likely have made both allies and enemies.
Note that these adventures do not involve the 'delving' for which Ptolus is famed amongst the adventuring community - but it's perfectly possible to intersperse the odd underground trip between the events presented here. A few suggestions are made for places to visit and even where to pop them in. However, there is plenty of action in the adventures, and those looking for a fight will not be disappointed.
Everything is cross-referenced to the rest of the book/other PDFs so it easy to look anything up when it appears in the adventure text - places, NPCs, and so on. The core four adventures involve a convoluted tale, a plot that strikes right into the middle of Ptolus affairs at the highest level - heady stuff for a bunch of inexperienced adventurers who may be new to the city or who have hitherto lived normal lives on its streets.
A neat aspect is the foreshadowing that goes on particularly in the first adventure, with references to things that will occur and people that will be encountered later on. They are not blatently obvious, but when the event happens or the individual met, the party should recall them. The plotline is wonderfully convoluted with myriads of people all following their own agendas - very lifelike in that respect, which added to the depth of background really makes the alternate reality come to life. It does mean, however, that careful study of the entire storyline is necessary to run it to best effect - this isn't something you can just pick up and play without preparation.
By the time the party has completed these adventures - including, if you wish, the several side-adventures mentioned throughout the text - they will not only be fourth-level but will be up-and-coming members of the adventuring community, beginning to make a name for themselves in Ptolus, and knowing a wide cross-section of people. Even if reading about the city has given you plenty of ideas for your own adventures, it's well worth considering these as a part of your ongoing campaign.