What are some good guitar and piano duets two guys can do together? Both can sing.?

So our IBA banquet is soon and I was asked, along with a friend of mine, to put on a small performance. I play the piano and my friend plays the guitar and we can both sing relatively well. (although I do admit he is better than me) Do you have any songs to suggest that we can do as a duet? Thanks for all your help!