The Top Secret Area can be accessed through Donut Ghost House on the second island of Super Mario World. The Top Secret Area contains 5 yellow boxes. The first 2 are Fire Flowers and the second 2 are Feathers. The middle box contains Yoshi. If you go to the Top Secret Area and are small, the first box you hit will be a Mushroom (with the exception of the middle box). If you go to the Top Secret Area and already have Yoshi, you will get a one up mushroom when you hit the middle box. If you think you need to skip this part of the game... Don't! It's very handy where you're running low on lives or need a cape/flower power. (Repeatedly enter this Secret Area when you have Yoshi to aquire more lives). This basically is the coolest part of the game.
When you leave this level in 2-Player mode, it's still your turn!