A small secret that is in the game is the ability to get two different difficulty levels. These levels are usually just Easy, Normal and Hard, but with Challenge Mode there are two added difficulties to unlock. Normally these are unlocked by completing Hard Mode, but there is a way to unlock them early..

If you hover over the Hard Mode on Challenge Mode and press Select & A, it will unlock the Super Hard mode. If you then hover over Super Hard Mode and press Select & A, it will unlock Intense Mode, which is the maximum difficulty.