Super Mario World/Top Secret Area
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 two on the left contain Fire Flowers while the two on the right have Feathers. The middle box contains Yoshi. As with all other boxes in the game, hitting the Feather or Flower boxes while small will yield a Mushroom and hitting the Yoshi box when you have a Yoshi will yield a one up mushroom.
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 acquire 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!