How do I force OS X to treat a file as a Folder?
I created a folder with two .jpg files in it, and renamed the folder "Today.Playground". As you say, it got associated with Xcode. (Nifty icon!)

So I changed the name to "Today.layground". At first, Finder wouldn't show me what was in it. (That is, I was viewing it in column view, and column to the right showed a preview of a generic folder, instead of listing what was in it.) I clicked on another item and then clicked on the folder again, and Finder properly showed the two .jpg files still inside.

It appears to me that that's all you have to do: look away and then look again.