Everybody loves JuK and/or Amarok. However, one hidden gem of Noatun is usually all I need to impress friends with KDE. This old feature (dated back to KDE 2.2 time) never failed me.
Launch Noatun, then choose menu Settings, Configure Noatun. Click Plugins icon from the list on the left, continue to tab named Visualizations. Now make sure entry Madness is checked and then click OK.
Open few windows just to populate the desktop. Harmless things like some folders opened in Konqueror will be fine.
Pick an interesting song which suits your audience well, play the song in Noatun, and see how they are stunned watching all those dancing windows!