A <a href="http://katlas.math.toronto.edu/wiki/Main_Page">knot table</a>, with the knots in it made out of a nice (pretty and pliable) material. It's aesthetic, and people might have fun playing with them.<br>
One might include also the <a href="https://mathoverflow.net/questions/879/most-interesting-mathematics-mistake/9059#9059">Perko pair</a>! They come with a story, and it's a lovely (terribly difficult, but tremendously fun) challenge to figure out how to change onto into the other.