If you press the triangle button while locked-on to an enemy, that will guarantee Donald & Goofy target that enemy. If you're not locked on to anything, when you press the triangle button, Donald and Goofy come over to wherever you are (in battle).
You can keep the Pot Centipede in just the first area of the boss battle if you keep it from forming a body enough times. It can also break open so that all the areas in the city are open during the boss fight (plus 20 billion pots from all over the place).
When you go back to the rabbit hole at the beginning of Wonderland (where Sora and co. fell down at the beginning) and fight all the Heartless there, a chest will appear. I think you can do this just after you've locked the Keyhole there and then again after all the new Hollow Bastion Heartless appear everywhere after defeating Rikunort. Also, in Wonderland, in one of the versions of Bizarre Room where the candles are on the floor, you can cut the wire/whatever it is underneath the purple pedestal on the wall, and the teddy bear will fall into the chair. If you go back to where you fought Trickmaster after doing that, you can swap the bear back in forth to bring the book or the clock into physical form. You get an item from the book, but you can move the clock to the side to reveal a shortcut (I can't remember to where exactly).
In Neverland, if you have Peter Pan in your party before you fight Antisora, when you get to the two doors, if you go to the one where Wendy and Kairi were supposedly at, it has a scene with Peter hitting the door. I didn't notice that for a while, because I rarely had Peter in my party (because I hate him) and just went into the door with the Trinity to get to Hook's cabin.