the cacklebat/ dark magic thing on shantae and the pirate curse was a nice way to make the player explore the game, the bad side is that it was needed for fighting the true last boss.
basically you have cacklebats throughout the game that are more or less hidden, and need specific abilities to be discovered, you fight them and collect dark magic upon their death, collect all 20 and you can access the real ending and final boss forms.
Warioland 3 had note coins in each level, find them all in the game and I think you unlock something (another minigolf challenge I think)