Press A + B + Select + Start during game play.

Successfully complete the game and save the game. An option to play the bonus games will appear when selecting that saved game from the title screen.

Successfully complete the game with a 100% rating.

Successfully complete the "Extra Game".

Defeat the Boss, then use the last Mike to hit the bonus plunger or piece of the star rod.

Just before you hit a star pad (the pad that reveals some secret level, warp, game,etc.), play the third song on your Mike. If done correctly the screen will glitch for a split second and you will have unlimited Mike usage.

In world 1 level 2, grab any power and find the Warp Star. Walk away from it so it is just out of sight, then release your power with Select. The warp star will be gone and you can walk to the end of the level, where "HAL" will appear.