Just a rough idea I had - similar to the rhythm schtick piece.

People on stage each with a different coloured piece of cardboard and assigned to a section of the crowd. (About 5 sections)
We one by one assign a note to each colour. (A rough idea of how this might work could be found here .. http://www.youtube.com/watch?v=ne6tB2KiZuk)
Then, proceed in guitar-hero styling to get the crowd to sing a song together (I was thinking something nerdy but simple.. star wars maybe? a popular video game theme?)
And then if possible .. act out a piece from whatever song we were doing on the stage.. like .. if it was star wars .. darth vader walks out or if it was mario we have him jumping about.

As you can see, a very rough idea - but I thought it was cool.