Use the adjective playful to describe someone who likes to have fun and doesn't take things too seriously. A playful kindergarten teacher makes everything seem like a game, even lining up to go outside for recess.
A group of cheerful kids playing musical chairs at a birthday party can be called playful, and so can a litter of wiggly puppies. A playful joke might be mischievous, but it's meant in a silly, friendly way. This word has been around since the thirteenth century, in the sense of "full of play." The Old English root word is plegan, which means "frolic, move rapidly, make sport of, or play music."