Super Mario World/Forest of Illusion 2

When you are at this level and have beaten it, you can go back and get to the blue switch. What you need to do is to go about half way, to the part where there are two spikey fish that just move up and down on their own. When you get to that part there will be lots of sleeping fish.

You have to maneuver around them and move almost all the way down until you see a yellow box with an exclamation point. The box may be guarded by a sleeping fish. You have to just keep going straight to the left of the screen until you see a key and key hole. This will open the blue switch!