The easiest way i found to do this was to stack a whole lot of romantic interactions including flirty jokes to get the bar high enough for the option to serenade to show up, at first it told me something about it not being high enough, so after some more rounds of flirty jokes and various interactions the option to serenade filly showed up.. I was going to do the shower but didn't want to take the chance the npc my sim was trying to woo would leave before my sim could pluck up the courage to ask for a date.