Passwords to start with everything
On the password screen type these codes in

C4TB RSSH 6RXC 1TJH
CUTK 3NFT YWMC WJVU

Sixteen of all items
Hold Up/Right + A + B on controller two and Down/Left + Select on controller one at the title screen. With all those buttons held, press Start on controller one. A sound will confirm correct code entry. Select Roas (the boy character), move outside to the tree shop. Try to buy a crystal, then return home. Select Roas again, move outside, and use all available magic energy. Return home and position the finger of the on-screen pointer on the eyes of the picture hanging on the wall. Hold Left + B on controller two and Right on controller one without moving the pointer. While all those buttons are held press A(16) on controller one. Sixteen of each item, except crowns and the dragonslayer sword, will be available.

Music test
Move the pointer to the painting on the wall in the house at the start of the game. Press A on controller two to hear various game music.

No damage on spikes
Hold Up while walking on the spikes.