Across the game are 139 Mini Medals - by collecting them and consigning them to Princess Minnie you can obtain some valuable rewards.

Reward	How to unlock
Dangerous Bustier	Collect 99 Mini Medals.
Dragon Claw	Collect 115 Mini Medals.
Fishnet Stockings	Collect 28 Mini Medals.
Flail of Destruction	Collect 121 Mini Medals.
Gold Nugget	Collect 52 Mini Medals.
Jester's Outfit	Collect 110 Mini Medals.
Kaleidoscope	Collect 127 Mini Medals.
Madarin Dress	Collect 133 Mini Medals.
Metal King Helm	Collect 90 Mini Medals.
Meteorite Bracer	Collect 60 Mini Medals.
Miracle Sword	Collect 68 Mini Medals.
Orichalcum	Collect 83 Mini Medals.
Posh Waistcoat	Collect 36 Mini Medals.
Sacred Armor	Collect 75 Mini Medals.
Staff of Divine Wrath	Collect 45 Mini Medals.