The Revenge of Shinobi/Secrets

Unlimited Shurikens
In the options screen: select 00 shurikens and wait awhile. Eventually the 00 will turn into an infinity sign and you will have unlimited shurikens in the game now, although it will say 999 on the counter.

30,000 Bonus Points
To earn a 30,000 point bonus, end a level with eleven times more shurikens than the number of lives you have remaining.

Extra Lives
One the second stage of level four, fire at the base of the first conveyer belt. A special Musashi symbol will appear. Hop onto the conveyor belt and let it carry you to the end. You'll lose a life but you gain two. Keep doing this until you have as many lives as you desire.