In Keltwick, when Bearwulf gives you the dungeon key to get to Malaga, sell it. It's worth 1000 kims and Bearwulf has an unlimited amount of them. Just go back, talk to him and he'll say ''Did you lose the key? I have another one.'' He'll give it to you, and you can repeat the process as much as you want.
Super Strength and Virtual Invincibility
For this cheat, you need a cursed weapon/suit of armor. Also, having Rafael's Stick helps save some cash, but you don't absolutely need it. Also, if you want the effects to be permanent, your level has to be maxed at 31. Equip the cursed weapon/armor. Now, either use Rafael's Stick or talk to a priest to remove the curse. Your strength or armor class (depending on what you equipped that was cursed) will go down. Keep re-equipping and removing the cursed item until your Strength/Armor Class goes to a ridiculous #, like 9999 or something like that. Now, re-equip your most powerful weapon/armor, and you'll either kill enemies in one hit guaranteed (Super Strength), or take only 1 damage from any attack from any enemy, including bosses (Invincibility).
Test Menu
During any time during the game, simultaneously press and hold the A, B and C buttons and press Start on controller 2.
A test menu should appear with Input, Sound and C.R.T tests. And when you exit, it will send you back to the Sega logo screen.