Invincibility:
Choose the "S.E." selection on the options menu. Press B to play the "He Runs", "Japonesque", "Shinobi Walk", "Sakura", and "Getufu" songs (in that order).

Unlimited weapons:
Set the "Shurikens" selection on the options menu to 0. Set the "Sounds" option to "Shuriken". Highlight the "Shurikens" option again and the 0 will change into the symbol for infinity.

Bonus points:
Complete a level without throwing any shurikens to receive a 30,000 point bonus.