World 7-4 (New Super Mario Bros.)
From the Super Mario Wiki
This is a very short level, and it consists of many mushrooms and Spin Blocks. It is similar to levels such as World 1-3, but it is a completely vertical level. There is only one enemy in the whole level, a red Koopa Troopa. The first two Star Coins are above mushrooms. The final one is within a trail of coins near the end. There is also a hidden area with more Spin Blocks and a P Switch, giving the player many coins. After the hidden area is the flagpole.
The secret exit requires Mario to be in his Mini form. The player needs to hop on a Flying ? Block, then around a pillar to reach a mini Warp Pipe. Going into the pipe will take Mario to the secret exit, which opens a path leading through the Green Toad House, all the way to the castle.