Make an iPod Screensaver
Here is a quick and easy way to change what your iPod video displays while it is charging. From web.mac.com, they have some good screenshots there as well.
Create a folder named Demo Mode.
In iTunes, rename the video you want the iPod to play.
After the iPod is on the charger two minutes, the video will start playing.
This only works on fifth generation Video iPods.
The iPod must be on pause and charging.
You must have enable disk use enabled.
the iPod video must have up to date firmware.
You could probably do this without updating the firmware, if that sort of stuff scares you. Try it first, if it works, why update it?