A lot of traditional songs have simple melodies and chord structures—songs such as “Down in the Valley,” “Darling Clementine,” and “Oh, Susanna.” They’re easy to learn and fun to sing, too, even if they’re not top-40.
@gailcalled, you need some minor chords for those first ones, and for “Puff” you need D7 and G7 (assuming you’re playing in C) to make it sound right.