Treasure Airship
In Worlds 1, 3, 5, and 6, you can turn the wandering Hammer Bros. enemy on the map into a treasure ship loaded with coins. To do so, you must complete a stage while the following is true:
The stage timer has an even number of seconds remaining
Coins collected are a multiple of 11 (i.e. 33, 44, or 55)
The 10s digit of your score matches the digit present in your coin total (for example, you have 11 coins and the 10s digit of your total score is a 1)
If all of the above is true as you collect the card and complete a stage, a Hammer Bros. enemy will turn into the treasure ship, and you have one attempt at the stage. You can collect a bunch of coins, and will face an enemy at the end.
White Mushroom Houses
You can make secret white mushroom houses appear in each of the first seven worlds by collecting the required number of coins in the corresponding stage, as indicated below.
1-4 - 44 coins
2-2 - 30 coins
3-8 - 41 coins
4-2 - 22 coins
5-5 - 28 coins
6-7 - 78 coins
7-2 - 41 coins
Your reward in odd-numbered stages will be a P-wing, while your reward in the even-numbered stages is an Anchor item. You will typically have to work really hard to collect every last coin required in the stages, as some may disappear if you don't make the best possible use of a P switch.
