Someone who tends to be happy and upbeat is lighthearted. When you are shopping for greeting cards, the lighthearted ones are the ones with the corny jokes, not the ones with pictures of dramatic sunsets.
Words like joyful, blithe, and happy-go-lucky capture the sense of lighthearted. People can be lighthearted, and so can entertaining things, like lighthearted movies, your uncle's lighthearted teasing, or a lighthearted family sing-along. The "full of cheer" meaning of lighthearted dates from about 1400, from both senses of light: "bright" and "not heavy."