1. Haven't tested celebrity - it's well known for its classiness and tends to attract older crowd. Group it with Holland America for the time being.
2. Midnight buffet used to exist. It no longer does as far as I know. Unless you cruised before, it might as well be but a myth now.
3. Dances/Clubbing, karaoke, scavenger hunts, swimming under the stars, mini golf, basketball, there's a whole variety of things. There is no obligation to attend any particular event