If you get enough coins in certain levels, you will find a White Mushroom House back on the map. Here are the minimum amount of coins you need to pick up at the following levels:

World 1, Level 4 -- 44 coins
World 2, Level 2 -- 30 coins
World 3, Level 8 -- 42 coins
World 4, Level 2 -- 22 coins
World 5, Level 5 -- 28 coins
World 6, Level 7 -- 78 coins
World 7, Level 2 -- 42 coins

The White Mushroom Houses in Worlds 1, 3, 5, and 7 give you P Wings, while the White Mushroom Houses in Worlds 2, 4, and 6 give you Anchors.