The Krusty Krab Playset is a play set that SpongeBob plays with in the episode "Bummer Vacation" when he couldn't think of anything else to do. It also has a mini version of it inside of it that's exactly like the playset except for the size difference.
It looks exactly like the Krusty Krab. It is unknown if it's made of metal, wood, or plastic. Though it is most likely made of plastic.