We were on the Dawn for Valentines week Feb, 2000. Valentines day happened to fall the second day of our cruise so I cannot say for sure if Valentines is always formal or not. In our case, we had:
Captain's "Love Boat National Holiday" Cocktail party
Honeymooners Cocktail party
Valintines Single Party
Valentines Poetry Contest
Hearts card game
Newlywed and not so newly wed games
Renewal of Marraige vows
Valentines Scavenger Hunt
MOst of these activities are found on any sailing.
They did have special menus printed for Valentines day and after the meal we were given a copy of the menu for a souvenir. The food choices were probably like any other Monday on a cruise.
They did sell t-shirts with a big heart on them and said that these shirts were only sold during the Valentines week. It was our first Princess cruise so I cannot say if that is true or not. Will find out in January when we take 2nd Princess cruise.