When making any work of art, there are always those ideas that are had but never fully implemented. Sometimes that's for the better. The above video shows some of the concepts that didn't quite make the cut for the first Super Mario Galaxy game. As is often the case, the unused elements remained in the final product's game code from whence they have been extracted for your viewing pleasure.
The bathtub planet is strange and you don't really get a great look at it in the video. The enemies are pretty cool, though. Particularly that one who looks a lot like Vectorman (or, if you want to go the bad route, Ballz).