活泼 describes a person or animal's cheerful, energetic personality — the natural liveliness you see in a playful child or a kitten. 生动 (shēngdòng) is for vivid descriptions or stories, not people; 活跃 (huóyuè) means 'active' in a group or field, like being a 活跃分子 (active member). The chemistry sense 'reactive' is a different, specialized usage.