How to make cakes/scones that won't go off, for display purposes
We used to make "display bread" using salt dough. Basically you add as much salt as flour (eg. 3 cups) to make it uninteresting for bacteria. To make the end result hard enough to last, you can add either hot water and a little oil or cold water and a spoon of wallpaper glue. Add as much water as necessary to form a bread like dough. Add anything (eg curry, blueberry sirup, ink instead of water - anything goes since you are not going to eat the result!) to color (parts of) your dough. Wholemeal instead of white flour will give you a stone or concrete like color.
Do not use yeast or anything else that will change the shape of your dough. Yeast probably will not work in all that salt anyway. 

Then shape your cakes and let the result dry for a day before baking it. Not in direct sunlight (it might dry too fast and crack).

To see some results, search for "Salzteig" on German Google Image search. :-)