Extra lives
Press Start to pause game play, then press Select(16). If done correctly, a blinking square will appear around the last digit of the number of your remaining lives. Hold A + B and then press Left, Up to change the number.
Find destinations easily
Once a level has been completed, if an orange ball spins around a location, you should go there first. If it does not do that, go to the temple.
Free Wario Golf play
Before you enter a Wario Golf game, save the game. Reload the game and enter the door. If you do badly in Wario Golf, shut the Game Boy Color. Reload the game and you will be outside the door. Keep repeating this until you win.
Extra lives...