There’s another thread with a handful of other ideas, but one idea in there is kind of related to yours:
For your use case, would you still want activities limited to only the cities you select?
The nuance between A) “particular geographic region” and B) “select multiple cities” might be that …
with A you’d get all your activities that fell within a selected rectangle
with B you’d get activities clipped to within the chosen city borders (an analogy would be a cookie cutter)
A simplified scenario might be that there are 4 cities with perfectly square borders that all fit together to make another larger square & you’ve completed the top left, top right, and bottom right cities
… in the “particular geographic region” version, if you selected a square on the map that covered your 3 completed cities, you’d also have any activities in the bottom left city in the poster
… in the “select multiple cities” version, if you selected top left/top right/bottom right cities, you would not have any activities from the bottom left city in the poster