He sat and played in the sandbox, where Slime Princess found him and mistakenly thought he was a new king.
Soon a number of princesses found out and mistook him for a handsome "Nice King," giving him the chance to capture them without their knowing or caring. By the time Finn and Jake arrived, Nice King (Ice King) had built a castle and prison out of sand and all the objects and resources around. The inside of the castle is made of sand, and has purple tentacles coming out of the wall.
The Park is full of sand, like a children's playground. It has a swan pond, a jungle gym, slides, play towers, and other well-known playground structures and play areas.