something with just the piano and vocals only. no drums or bass. maybe one with the electric or acoustic guitar instead of piano, something that was a classic that got regular radio air play at some time. any era really.

1985 - Wings (there are other instruments as well but i think it sounds fine with just a piano)
Imagine by John Lennon, Someone Like You by Adele, Candle in the Wind 1997 by Elton John (or most other Elton songs) are probably the three most famous ones that are based primarily around piano and vocals.

'Karma Police' by Radiohead works well with piano+vocals, Thom Yorke did a live performance of it on the Jonathan Ross show. There's also a great piano+vox only version of David Bowie's 'Life on Mars'? up somewhere on YouTube.