World 5-A (New Super Mario Bros. 2)
From the Super Mario Wiki
World 5-A is the second of two secret levels in World 5 in the game New Super Mario Bros. 2. The level is unlocked by completing the world's other secret level, World 5-. The level is an auto-scrolling level that mostly takes place on a ship flying through the sky.
The level starts with Mario on a platform, with a flying ship that travels through the level directly underneath it. Between the first and second sets of blocks is a platform with a Koopa Troopa on it, and directly above the second set of blocks is a Gold Ring. The next platforms have a Koopa Paratroopa and another Koopa Troopa on them, respectively, and between and after the platforms are two more Gold Rings. Afterward are a pair of horizontally-flying Koopa Paratroopas and another two platforms with a Koopa Paratroopa on the latter. Afterward is a Roulette Block with coins in it, and above that is another Gold Ring. The next platform has a Springboard atop a pair of Brick Blocks, and beyond this is a pair of platforms, the lower one with the Checkpoint Flag, and the upper one with a Gold Ring over it; on both sides of the platforms are Koopa Paratroopas. On the next platform are several blocks, one of the ? Blocks containing a Super Leaf. The next section has several higher up platforms, the first with a Gold Ring and the other platforms containing coins (one with a Star Coin); between some of these platforms are Koopa Paratroopas. The next section has coins, Gold Rings, and horizontally-flying Koopa Paratroopas above the ship. Directly after this, the ship comes to a stop near the Goal Pole.
At the end of the level, there are two brick blocks. While the one on the left contains a coin, the one on the right contains a Beanstalk. Use the springboard found earlier in the level to make it easier to reach. Reaching this exit unlocks the path to a green Toad House and World 5-.