What song should I sing at my Grandparents' 50th anniversary?

I am a professional singer and I want to sing at my grandparents' big anniversary banquet. I also think I might have a girl sing with me... That being said, I'm having trouble picking the song. I want a really good song with a really good melody. I want a song that I can really show-off on... After all this is my grandparents' 50th, I want it to be memorable! So a nice song, maybe a duet. Any suggestions?