Usage patterns
快活な〈人〉a cheerful, lively person
He's a cheerful person who talks easily with anyone.
快活に〈動詞〉to do something in a cheerful, lively manner
She greeted us cheerfully, laughing all the while.
快活な笑顔a bright, cheerful smile
Seeing his bright, cheerful smile lifts my spirits too.
快活な性格a cheerful, upbeat personality
With her cheerful personality, she's popular in class.