In Super Mario Bros 3, how to make power-up items appear in Hammer Bros battles

To make a Hammer Bros. turn into a coin ship:

1. Ensure there's a Hammer Bros. left on the world map
2. Start a level and finish with:
   1. A multiple of 11 coins (11, 22, 33, etc.)
   2. A score ending in 10 (2010, 3410, etc.)

Each second left on the clock gives you 50 points. That means you should end with an even number of seconds left on the clock if you have a score ending in 10. If you have a score ending in 60 you should finish with an odd number of seconds on the clock. Anything else and you're out of luck!

There are also white mushroom houses that appear if you get every coin on a particular level in each of the first 7 worlds:

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

There were two hints that the warp whistle was there, and both of them were contained in letters from Princess Peach.

Between worlds 3 and 4 you get this message:

The White Block contains magic powers that will enable you to defeat your enemies.

And between worlds 6 and 7 you get this one:

I am well. Please retrieve the Magic Whistle hidden in the darkness at the end of the third world

Although it's not a map and a sign, both hints together are enough to point the way to this particular whistle at 1-3. It looks like a basic way to reward a replay as you made it through the game. There are even less clues for the other whistles; one other letter makes reference to "east of the sand dunes" for the desert world whistle, and for the first fortress whistle, there's nothing to hint at it other than Super Mario Bros hid a warp zone in exactly the same way (plus as mentioned by Ryan, it was practically advertised in a movie).

Otherwise, intentional vagueness and cryptically hidden random-access secrets (like white mushroom house, and pirate ships full of money) was a good way to sell video game guides in the early Nintendo days.