Classic NES Series: Metroid

Enter 999999 999999 KKKKKK KKKKKK as a password with no Varia suit and a different ending.

Use the Ice Beam to freeze the center spike in mid-air as he shoots. Then, use your missiles to shoot the bare spot where the spike was located.

Complete the first quest without dying and with all the items (ice beam as your gun weapon). Press Start after the credits have completed. If you have completed the game correctly, Samus will take off her suit in the ending. You will start with all your items you gained in the previous quest minus your missiles and suit in the next game.

Press Start to pause game play, then press Select + Up to display your current password.

Go to a pipe that has enemies constantly appearing at ground level with no other enemies around. Just keep shooting and the enemies will fly into your bullets, leaving energy and other items.

In various parts of the game, you may find yourself blocked or cut off. You are either stuck or you can try shooting the walls around you. If you are lucky, one of the blocks will break. If it does, continue shooting until you cannot shoot further. Then, jump and try moving to the left or right. There is usually a ghost wall that you can go through. This trick is useful for gathering power-ups and finding secrets.