Alex Kidd in Miracle World/Codes

Continue
If you lose all your lives, but have at least $400, you can continue. On the game over screen, hold up on the D-Pad and rapidly press Button 2 eight times. Using a continue costs $400.

Janken matches
Note: This order may vary if Alex loses a match or gets a draw, so make sure to win all matches.
 * 1st: Rock, Scissors
 * 2nd: Scissors, Paper
 * 3rd: Rock, Scissors
 * 4th: Paper, Paper
 * 5th: Rock, Rock
 * 6th: Rock, Scissors
 * 7th: Paper, Paper

Question mark blocks
All blocks with a question mark contain items that appear in the same order: Power Bracelet, a ghost, and an extra life. The start of the order varies between stage, so before breaking one, make sure to have an escape course in case a ghost appears.

On the first level, the first question block broken will grant the Power Bracelet, in the second a ghost will come out after Alex (so run from it) and the next will be an extra life.

Secret passage
In the third stage, defeat the first octopus and press down while above its pot to enter a secret passage.

Final stage
The secret code shown in the Hirotta Stone is: Sun, wave, moon, star, sun, moon, wave, fish, star, fish.