If you're a completionist, like me, or you want to win in Flick Rush, or you want to get a Trophy, or beat the superbosses, then you probably have been hunting for materials to make better spirits. Some of the ones that catch people's eye are things like Brilliant Fantasy and Epic Fantasy that you can't seem to find anywhere.
Well, you can, through the use of Special portals (told by the forecast), which show you a spirit, and beating that challenge usually nets you a Dream Piece. All my info is from KHInsider, so check it out if needed.
Secondly, while it won't give you items, for challenges and HP boosts, there are secret portals in most of the worlds right in the area where Riku and Sora fight their respective bosses. You re-fight them here, but they are much stronger and give you an HP bonus when you beat them, perfect preparation for the final Superboss, Julius.
Just wanted to give some people a hand.