Ghost Ship level:
Beat the game and save. When you load the cleared game data, the Ghost Ship can be reached on the Galaxy map. Beat it to get the Dolgen Edge and unlock the Captain's Clothes.
Ghost Ship Extreme level:
Answer all the Seventh Mystery questions in the Ghost Ship to get the Key of the Underworld. Use the Key of the Underworld to access the Ghost Ship Extreme level. Beating it gets you the Truth of the King and unlocks Kisara's Swimsuit.
See treasure chests on the Minimap:
To see the locations of treasure chests on the map, activate every teleporter on that planet. You will receive a message when you walk through the last teleporter. Treasure chests show up as yellow dots on the map.
The Earth key is found in the elder's hut in Burkaqua village, the Star key is outside Angela's bar on Myna, and the Sun key is in the old man's house in Johannesberg.
Thanks to Revolution Readers tinyhat, Wilton20k and Xemperor!