Choose the "Name Entry" selection at the options screen. Select "Yes" to enter a name. Do not actually enter a name. Instead, press (L) to display the music test screen.

During a break when it shows a character, press (A) and something will happen.

Successfully complete the game to unlock an option to fight just Bosses.

More Unlockables for Wario Ware Inc: Mega Microgames:

You can scroll through the writing faster on the grid by tapping or holding (L) or (R).

Press (L) while the credit music is playing to increase the bass, or press (R) to increase the treble.

Hold Left, Right, Up, or Down during the credits to change the star patterns.

Eat a white seed to refill one of the holes in the ground, then eat a flashing seed to refill all of the holes and make all the seeds on the screen disappear. When you catch a seed when it is very close to the ground, you will get 10 points. When it is a bit farther off the ground, you will get 50 points. When it is pretty far off the ground, and you will get 100 points. When it is very far off the ground, you will get 300 points. When it is at the very top of the screen, you will get 1000 points.

During the introduction of a level, turn off the Game Boy Advance, then turn it back on. Go to the level that you were just on and you can now press (A) button to skip the introduction.