Instant Emerald (Level 1)
To get an emerald in the first level, press UP, DOWN, UP, DOWN, 1, 2, 1, 2, 1, 2 at the Sonic/Tails selection screen.
At the Main Title Screen, enter UP, UP, DOWN, DOWN, RIGHT, LEFT, RIGHT, LEFT, START. It might take a few times to get it to work.
At the title screen, press Down, Down, Up, Up, Left, Right, Left, Right, 1, 2, Start.