New Super Mario Bros./World 7-Ghost

Even in the clouds, one can find a decrepit mansion haunted by Boos. Many doors lead to different rooms, making you walk down hallways and find switches only to end up back where you started. Explore every room thoroughly so that you don't miss any secrets. Do not fear when you see ethereal hands floating above, as they are the key to navigating this odd mansion.

Level walkthrough
You start in the mansion's main hall, which serves as a hub of sorts for the majority of the level. Hit the first block from the left to make a switch pop out. Hitting the switch causes five doors to appear around you, and going in these doors is what you'll be doing for most of the level. Even if you're going for total completion, you can feel free to ignore the bottom-left door, as it only contains coins. The door to the right also lacks any collectables, but it does introduce the Phantom Hands, floating hands which all point to a specific spot. If you jump where the hands are pointing, you can hit a hidden block containing a power-up.

Of the three remaining rooms, two contain Star Coins. First, go into the top door to arrive in a room filled with an odd yellow goo. The goo is usually solid, but you can depress it by ground pounding the surface, after which it slowly reverts to its original shape. Position yourself above the Star Coin in the goo and ground pound until you can collect it. Do the same with the switch for the door to return to the previous room. Next, go into the left door. The room it takes you to also contains goo, this time moving in a wave pattern. You want to jump from the crests of the wave to reach the second Star Coin suspended in the air.

Now that you're done exploring the other rooms, it's time to move onward. Go into the one door you have yet to enter, which is the bottom-right door. You will end up in a mostly empty room with only a spring and a door that leads back to the main hall. However, there are also a lot of Phantom Hands floating above you. If you've been to the right door from the main hall, you might have already figured out their purpose. These hands all point to an invisible block, which moves back-and-forth in the room. Hit the hidden block to make a switch come out. Hit the switch to reveal a moving platform. Use the spring and the platforms to get to a ledge near the ceiling, where a different door will take you to the final room of the level.

There are tons of coins around you but no floor, as well as a single P-Switch up above, so it's quite obvious what you have to do. Hit the P-Switch to turn the coins into platforms, then start running to the right quickly, jumping on the newly-formed platforms. While you're safe once you can see the ground beneath you, you might spot more brick block platforms leading upward. Jump up the platforms before they transform back to get on to the ceiling. Run to the right to find the last Star Coin. After this, fall back down to the floor and run right to reach the exit door. The flagpole is right outside.

Star Coins

 * 1) Go into the top door from the starting room and ground pound the goo to reach the Star Coin.
 * 2) Go into the left door from the starting room. The Star Coin can be obtained by jumping from one of the crests of the wavy goo below.
 * 3) Run right after hitting the P-Switch. When you see some brick block platforms forming steps upward, jump up them to get to the ceiling. Run right to the end for the Star Coin.

Alternate exit
After hitting the P-Switch, try running to the left instead. As you pass by some brick blocks overhead, look for one block that has a glimmering effect. This signifies that the brick block is actually a disguised question mark block, and hitting it causes a vine to grow. Get on to the block before the other brick blocks revert to coins, then climb up the vine to reach an alcove with a door. The door leads to the secret exit of the ghost house, where the red flagpole can be found.

This secret exit creates a shortcut to the World 7-Tower, skipping two levels while opening a green Toad House for you to use.