HAL Test Room
After you defeat the Gigant Edge mini-boss in Royal Road 1-1, head to the far right and fly up. You will find a path comprised of a bunch of stars. Follow that path to its end and look up to the sky, where you should spot a crescent moon. Fly up to the moon, which has an opening. Press UP to enter that opening, which leads to the secret room. You'll find the HAL logo built out of blocks, along with an enemy and several ability options. Destroying the blocks also nets you extra lives, making this room a great place for supplies.