Theme demos

This might seem a bit of a lame question.

What is the best way to show veiwers a demo of a particular WP theme in advance of them setting up a multi site and changing it there.

Like a kind of whats on offer showroom. The themes on here have a drop down menu for demo display, can I add something similar to my site? What runs that?

Otherwise all I can think of is a static image display.

Any ideas please?