Nostalgia

Enter a dungeon and fight enemies until your Rank Bonus reaches 99. Exit the dungeon and immediately go to high altitude. The first battle started will drop your rank, but you will almost always get you the rare drop for those enemies, which is usually a gadget. This will not work if you run from an enemy.

Attain 100% completion of the Adventurer's Notebook to unlock the Sage Ring.

Successfully complete the game. You can now access the Theater mode option in the Adventurer's Journal. It allows you to see the cinematics again and listen to the music.