Press L, R, and Start simultaneously to return to the title screen without saving.

Beat the final Bowser in World 8 to unlock the Special Worlds. On the map screen, enter the pipe at the beginning of the world to switch between normal and special.

You'll unlock Luigi after beating Special 1-Castle. Switch between Mario and Luigi on the map screen by tapping the "M" or "L" logo on the touch screen.

When Mario or Luigi is near an invisible block, they will look up at it as if it were a normal block. Watch for this hint and you can find lots of hidden goodies.