Lately Princess ships have been having "Princess Pop Choir", where a group of 15-20 people who love to sing practice with a Cruise Staff member 3-4 times during the cruise.
They give you lyrics to popular standards, easy choreography, and a lot of help.
Last evening of the cruise (maybe 5:00 p.m.), the choir performs in the Atrium. It's hugely successful and tons of fun for both singers and audience.
Watch the daily newsletter - I did it on the Ruby Princess last November and it was huge fun!