I always sing a hymn or Primary song to my three-year-old when I put her down for a nap. I usually ask if she has a preference, whereupon she asks for a song about painting (I sing "Primary Colors") or bed time ("Did You Think to Pray?" - "Oh, how praying rests the weary . . .").
Today she asked for a song about hangers, so I sang "Hold to the Rod." :)