Thank you for the review! I'm glad you enjoyed the game and the story!
There are a few unshockable secrets:
Bonus Beach Level - Rescue all Riwi on Lethea (13 I think)
Skimmer Race with Sierra - Complete both skimmer (the hover jet) levels.
VR Arena - Find code hidden in the game. (I think in the final level)
VR Training - Find code hidden in the game (I think in Lethea)
There are additional hidden codes that modify the items that spawn in Luna's ship.