Instead of immediately jumping onto the flag pole at the end of a stage, take your time and watch the clock. If you hit the flag when the last two digits of your timer are the same (example: 133, 211, or 099) you'll unlock a mushroom house on your World Map. If the last two digits are 1, 2, or 3, you'll unlock a red mushroom house. If the last two digitsl are 4, 5, or 6, you'll unlock a green mushroom house. If the last two digits are 7, 8, or 9, you'll unlock an orange mushroom house.