What's a retro mac without software? In fact, computers are not much use if all you have is the operating system.
So specifically, my suggestion is to find a User Interface feature or function in the current OS which is older than anyone thinks or one that does not currently exist in OS X!
James does not have to know about all this software. The Retromaccast community can contribute!
Let me give an example: Kineticon was an extension that provided animated icons. I don't know of any utility for adding this to OS X.
I believe A-Dock was software for adding a Dock to OS 9.
And so on. We have weeks of material if we think about it!