Infinite POW ball (Japanese Version)
Area 7 has a separate building on the upper right. Stand in the middle of the building and press "up", you can enter a hidden communication room. Communication will tell you to knock on the right wall 5 times. After do it, you will hear the sound like get 1UP, then you can use the POW ball indefinitely(press "A" "B" at the same time).
Obtain All Weapons (Japanese Version)
In Level 17's interrogation room, beat the prisoner 255 times and you'll receive all the weapons.