The secret: It’s debated whether Yoshi’s Island is indeed a “true” Mario game and not a spin-off, but it certainly retained the series’ love of hard to acquire secrets. That includes a number of harder stages, as well as versus modes that you need a special code to see. Yes, at present every single game seems to come with multiplayer whether you want it or not, but Nintendo in the mid-’90s made players work for it.
How to get it: Unlike the bonus stages that demanded 100% of a world’s collectibles, the Mini Battles just asked for a certain level of inquisitiveness. In World 5 midpoint Sluggy the Unshaven's Fort, you’ll find a seemingly purposeless room with a single Chain Chomp inside. Dispose of that guy and a door will appear, which leads to a room with a hint block saying, “This is Top Secret so LISTEN UP! On the Level Selection screen, hold Select and press X, X, Y, B and A!!” Sure, now you could skip all the trouble and input that code at the start to unlock Mini Battles, but where’s the fun in that?