Begin playing a Game Boy game in the Virtual Console. Then, hold L + R and press Y to change the colors from black and white to the classic green LCD palette. Repeat the code to disable its effect.
You can get more than 10 Play Coins a day by changing the date under the system settings. You can earn one PlayCoin by taking 100 steps with the 3DS in Sleep mode.
Once you get your 10 Play Coins for the day, go to "System Settings" under "Other Settings", and choose "Date And Time". Change the date to one day ahead, save the change, then go back to the current day. You will get another 10 Play Coins. Repeat this trick as many times as desired.
Have ambient music playing while the system displays a game cartridge at the home menu, and the cartridge will "dance" to the music that the system's microphone picks up.
When you blow into the mic, whatever is on the top screen on the main menu (except for "Health & Safety Information") should start spinning, and the title will go up.