Easy money:
To easily gain money, register five new survialists, and set all their skill points to Chop. Add them to your party. Enter the Labyrinth, go north twice, then east. Go though the shortcut. The shortcut must be activated from the other side before it is useable from this side. Next, go east, then north, and repeatedly press A until you reach your daily limit. After that, go back to town. Sell all the items, and sleep until morning. Repeat as desired.
Unlock BGM option:
Complete the game to unlock the BGM option.