Any encounters you do should plant the seeds for the events in Magnimar in book 2. As-written that section of the adventure is a little thin and needs a bit of work to bring it to life. Particularly, Ironbriar needs to be introduced, even if it’s just by name or reputation, as early as possible so that when he shows up later as a villain the PCs know who they are dealing with. Since your PCs live in Magnimar and you are starting the campaign there, you have a perfect opportunity to do so. Could be a simple encounter like the PCs chase down a couple of low level pickpockets in the market. They help catch them, and later are summoned to court (Ironbriar presiding) as witnesses.
But anything you can do to flesh out the relevant details and NPCs that show up later would go a long way. I second Kalshane’s suggestion to have your PCs overhear the Scarecrow rhyme. Mundane hints about Aldern Foxglove could be good, but he gets a pretty solid intro to him as-written, and focusing too much on him early could make your PCs suspicious, so I’d try to keep that in the background and mixed in with other leads or red herrings.
Besides that, a great source for simple random encounters that fit Magnimar is the Magnimar: City of Monuments campaign setting book, if you have access to it (you can buy the pdf on this site). There are encounter tables for each section of the city at different times of day. You can use these encounters as is; as simple, inconsequential to the narrative encounters, or use them as starting points to flesh out more in-depth encounters.
Good luck and have fun!
