So what follows is an activity that combines the metal insets with a little play of straight lines vs curved lines in order to create a 3D optical illusion. (This can also be associated with a discussion/lesson on the science behind optical illusion.) For even more examples of this phenomenon you can see here and here and here ...
So the first step is to choose a metal inset (or any other shape that can be traced on paper) and place it on a white piece of paper. Trace around.
With a ruler trace horizontal parallel lines on the page, excluding the shape you have just traced. This gives the child quite a bit of practice with the ruler, too :)
Now the tricky part: using the points of contact of the lines with the edge of the figure as a starting and finishing point, trace an arch joining these two points. Again, it does not have to be perfect, but this first line is going to be the guide for all the other curves inside the figure, so you can actually help the child by tracing a couple and explaining that we still need to keep them parallel.
Once all the lines are traced, you are ready to colour. I made my children create their own color pattern, but it can also be just random. They can get creative there.
After all the stripes are fill in with colour, you can enjoy the effect!
This is very simple, but quite rewarding, even as an introduction to the concepts of perspective and any type of "applied" optical illusion in art and architecture.