At the Sadko restaurant on Glinki Street, the waiting staff are all students at St. Petersburg's Conservatory (Vocal Department). In the middle of your meal, and for no reason whatsoever, they burst into song - not for money, not for tips, just because... it's lovely. Somewhat unsteady video below. (Excuse the abrupt ending - I slammed the camera down to join the other diners in rapturous applause...)
What a place.