if you hold "A" when you start the gamecube up (with the disc in) you can go to the Gamecube Menu and edit the Gamecube info (time, date, memory card data, etc..)
you can also have a PS2-Esque feel to your gamecube by starting it up with no disc in it, you can acess the menu freely and you can put the disc in at any time.

When the Gamecube is starting up hold the Z button to hear a different sound. If you plug in 4 controllers and press the Z button it will play a completely different sound.