Karaoke Night

Find a friend with a laser disc player or a Karaoke machine and rent some Karaoke Discs (screen the songs beforehand- a lot of the videos are pretty raunchy). If you don't have a professional Karaoke machine, you can hook up a laser disc player to a sound system and a 19″ TV that the person that's singing can see. Have the video input split so it is projected on a large screen behind the person.

If you have the ability, send the music through a set of earphones (so the person singing can't hear themselves too well).

If you use a video mixer and a camera, you can shoot the person singing and place them on the screen in a wipe mode or mixed in.

Hilarious Method:
Have the music play through the earphones only during a song or two so you only hear their voice out loud- always good for a laugh.