Super Mario World/Valley of Bowser 2

Secret Exit - Pass through the Midway Gate, come out the first pipe, make your way through the rising and falling ceiling maze part, and go through that pipe. When you see the first Mega Mole, don't jump down, just stay at the edge. The maze will not hit you, it stops behind you. Use it as stairs to the left onto the top of the ceiling. Continue to the left until you drop down to the Key.

Also, just before the falling ceiling maze, there are two Q-Blocks. The first (and visible) Q-block releases some Yoshi-wings for a secret bonus area, if you have Yoshi with you. Which is cool, as Mario riding Yoshi is too big to survive the ceiling maze. The other block is hidden off-view in the upper-left above the pipe you just came in through, and always has a feather.