To get access the hidden planet, get 5 Gold Bomber Hero Medals and all of the Other-Dimension Bombs.

Slider Race
By: MetroidMoo (MetroidMoo@aol.com)

To get the Slider Race Option, get 3 Gold Bomber Hero Medals.

Gold Bomber
By: MetroidMoo (MetroidMoo@aol.com)

To get the Gold Bomber option, complete the hidden planet.

Millian's Treasure Hunt
By: MetroidMoo (MetroidMoo@aol.com)

To get the Millian's Treasure Hunt option, get 6 Gold Bomber Hero Medals.

Level Select
By: alterpunk@hotmail.com via GameSages

Press A, B, and Z.. You will now have Level Select as an options.