I want to say that in a commentary track or somewhere, the prop that Aykroyd carries is revealed to be the plans for a house Ivan Reitman was building at the time. I might have imagined that, though.
And I think the John Decuir blueprints for construction of the sets for Dana's and Louis' apartments can be found in the book Making Ghostbusters. If you don't own a copy, check out the PDF scans of it from SpookCentral.
Alex