Gravitational lensing is one of the most interesting potential effects of dark matter. It provides evidence of a lot of physical phenomena including black holes and the hidden masses of the universe.
Claudio Maccone proposed using the Sun as a gravitational lens to peer around the nearby galaxy to a greater degree. It may be interesting to see how gravitational lensing could be faked. The most obvious way, and one often used as a metaphor is to use the bottom of a wine glass. The largest problem with connecting physics, especially cosmology and particle physics, to people who aren't ready for it is the problem of scale. I often find when discussing these things that 'household' objects and lots of hand movements can be really effective and help people to understand.
It's very much a mental engagement, not one easily achievable through objects or complex concepts. Just one that needs people to find and flick the switch in their head that says; 'OK, this is huge and awesome and changes everything.'
I think gravitational lensing is a fantastic metaphor for that.