If  an acapella version of "Star Spangled Banner" is not your taste, I've also done "Amazing Grace" and a Gregorian Chant acapella. 

Last year I showed up for a show rehearsal for which I was doing a solo version of "Danny Boy.  Unfortunately, no one showed up with the recorded music I was supposed to use and no one had brought a keyboard.  After a debate started over what to do, I interrupted and said that I would just do it acapella they said to go ahead and try.  After I finished and came off the stage, the music director (who had done hundreds of shows) stopped me and said that I was one only three people she knew that could have done that. I was way easier than walking out on stage in front of an audience of 500 paying customers with a dead mic (which has happened a couple of times). 

I guess I should add that my professional career had nothing to do with singing or stage performance.