Atomic Boo:
In the Creepy Steeple, if you hang out too long in the stained class main corridor, you get surrounded by all the itty Boos and get thrown out. If you let a bunch surround you, charge up your Hammer Spin, and whack enough of them, they cry out, morph into one, and attack. I don't know if your answer to the question that first Boo makes before it unleashes the mob will affect this sequence of events or not. And yes, Atomic Boo = Giant Boo.
8-Bit Sprites: In one of the rooms in the X-Naut Lunar Base (Sublevel 2?) you can Ultra Spin Jump through a loose panel in the ceiling to a walkway above. This walkway has a series of barred gratings that you can "thin yourself" through. One of them leads back to the room where you changed into an X-Naut costume as Peach, only you'll land on top one of the changing stalls. Jump into it, and if the drapes are closed when you jump into it, by the time you exit, you'll hear the classic SMB theme song and your crew will be 8-bit. Check out the rest of them in your party screen before leaving.