Since no one mentions this in their solutions:
At one point you might realize that you don't unlock any more items, because you might have unlocked everything that you can unlock by f.e. playing exhibition matches without bots.
I was missing only one item and that was the "MOAI" antenna. You can't unlock it via exhibition matches. Instead you will have to enter the KONAMI CODE at the screen that asks of you to "PRESS X TO START THE GAME".
The KONAMI CODE is as follows:
Up Up Down Down Left Right Left Right