Imagine a creature so big, so uncontainable and uncontrollable that it grew so large as to leave the planet it once was on in order to find more food/resources to sustain its hunger, floating through space from planet to planet devouring everything that bumped into it.
I think a planet designed to be one whole creature would look very interesting and would have some really cool elements to it, some dangers/problems you'd have to overcome could be:
1. Giant arms/tentacles on the surface of the planet that try to grab you when you get close.
2. Impenetrable skin which couldn't be