Unlimited shurikens:
Select the "Shurikens" choice on the options screen and set it to 00. After approximately ten seconds this value will change to an infinity symbol.

End bonus:
Complete any level with a number of shurikens equal to 11 times the number of remaining lives. 30,000 bonus points will be added to your score.

Hint: Unlimited lives:
Once you get to the level with all the mechanical parts and lava, jump in the air, do a flip, and shoot out a large number of shurikens. You should see an extra life box that actually has two lives. Collect it until you have 400,000 points and you will have unlimited lives. Note: Pause game play to view how many points you currently have.

Hint: Restore health:
If you enabled the "Unlimited lives" trick but are low on health and you still have your ninjitsu, use one to blow yourself up. Your life will be restored. You will not die and have to go back to the start of the level, and you will kill also everybody nearby. However, you will lose your power shurikens and any extra ninjitsu.

Hint: Save your girlfriend at the N. Master:
On each wall, there is a hole. Throw shurikens in them and the stone wall will stop for a few seconds.