Sword of Mana/Secrets

Niccolo's Special Menu
If you buy 251 items from Niccolo, he gives you access to a new menu where you can buy rare accessories and raw materials! However, this doesn't mean that you can just buy 251 gumdrops in one shot, you have to buy something, leave the menu and then repeat 250 more times.

The Chocobo
Complete the game, and leave the Game Boy on the 'fin' screen for a few minutes for a special surprise. No, you cannot use the chocobo.

Black Enemies
Beat 1000 of any enemy to turn it black, it will have a stat boost of 100 to everything except HP which has a 500 point stat boost