Whatever you do, don't learn Piano Man by Billy Joel. My bro who is a piano man in a hotel restaurant, refuses to take it as a request any more. That is how often it is asked for.

If you don't know it, you won't get suckered into playing over and over again.

Learn some unusual versions of birthday and anniversary songs. Learn some kids songs. My bro has a few stock Sesame Street and Disney up his sleeve, for when the patrons bring in kids.