There are a wide range of synonyms for the phrase "as playful as a kitten", which can be used to describe someone or something that is full of energy and constantly finding ways to have fun. Some possible alternatives might include "boisterous", "exuberant", "sprightly", "lively", "frolicsome", "bouncy", "frisky", "perky", "zestful", "spirited", "peppy", "animated", or "jaunty". Each of these terms conveys a sense of youthful vitality and restless energy, making them perfect for describing a wide range of activities and personalities that are full of life and mischief.