Honestly, there isn't a whole lot downtown for mystery shopping if you don't want to stray far from your hotel and America's Center. Are you staying at a Courtyard or the Marriott Grand directly across from the convention center? The MG is a beautiful hotel.
STL is like any other metro, some lovely areas, some not so much. Check out Ellis since they have had some loft apartments within blocks of where you are staying. You didn't say if the conference is on weekdays or the weekend. If you will have some week day availability, look for some bank shops. Off hand it seems Bare and Ath occasionally have some in the heart of downtown.
If you find anything just shoot me a message and I can give you the low down on the area.
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