Unlimited items or gold:
Find a location that contains multiple items or gold, such as
the first room in Oedo Castle. Collect the items, leave the room,
then re-enter. The items will be available for recollection.

Boss mode:
Collect all forty-five Fortune Dolls to unlock a boss mode
selection on the options screen.

Bonus picture:
Enable the "Boss mode" code, and complete the Boss sequence.

Control Impact:
Press the Analog-stick when Impact's introduction sequence ends
to move his head.

Rumble Pak bonus:
Although not an official Rumble Pak game, the Boss levels will
support that peripheral.