Catherine Zeta-Jones serenaded husband Michael Douglas and friends at Hollywood’s Sunset Tower Hotel by singing tunes from “A Little Night Music” at the lobby piano yesterday morning. A spy told us, “Catherine stood at the piano for an hour. She kept on belting out ‘Send in the Clowns.’ ” Zeta-Jones, in a skin-tight floral print dress and heels, also wowed a small group of hotel guests who “were stunned to see her, but then sat in the lobby listening.”