I did Nat King Cole's "Unforgettable" for my dance with my mother.  I'm surprised to say that it felt a bit long, even though it's only like 3:30.  My mom and I were both like, "Isn't this song over yet?" at the time.  I don't have any particular song suggestions (there are plenty of lists on the web about it), but I'd advise to keep the length of the song in mind.