Completion Bonus:
When you have completed the game you will be given a prompt to 'Save'. If you then continue from the saved file there will be a bonus dungeon.

Spawn Points on Desert Island:
During the night go to the bowl of the second desert island you can reach and you will find three spawn points there. One is behind the tent, another is next to a rock while the third is on the side of the cliff when you climb down.

When you have collected the indicated amount of skills or monsters during gameplay the commissioner's employees will give you your prize.

Unlock Metal Kaiser Slime:
Collect 150 Skills.

Unlock Metal King Slime:
Collect 100 Skills.

Unlock Leopold:
Collect ALL the Skills.

Unlock Liquid Metal Slime:
Collect 100 Monsters.

Unlock Grandpa Slime:
Collect 200 Monsters.

Unlock Empyrea:
Collect ALL the Monsters.

Unlock Robbin' Hood:
Collect ALL the Monster and Skill libraries.