I haven't yet met a woman who didn't appreciate Clapton's Wonderful Tonight.
Some songs will depend on whether the tape is for a wife or a girlfriend. Obvious choices (to my mind, anyway) that come immediately to mind include:
Beach Boys, Wouldn't it be Nice?
Harry Connick, Jr., It Had to be You
Sinatra, The Best is Yet to Come, The Way You Look Tonight
Marc Cohn, True Companion
John Cougar Mellencamp, Jack and Diane
Johnny Cash, I Walk the Line, Ring of Fire (probably played out, though)
Mamas and Papas, Dedicated to the One I Love, Dream a Little Dream of Me
Peter Gabriel, In Your Eyes
Smithereens, A Girl Like You
Stevie Ray Vaughan, Pride and Joy, Love Struck Baby, Lenny
Beatles, Love Me Do, I Want to Hold Your Hand, so many others...
Dire Straits, Why Worry
Don Henley, The Last Worthless Evening