A nice stroll around the deck can be romantic.
Dancing is always fun.
The hot tub idea sounds great.
Order room service one day & have breakfast in bed.
Money is a personal issue depending on your spending habits
do you drink? gamble? buy souveniers? buy photographs? play bingo? want a spa service? want to go on excursions? I've read $100 per day seems to be the "average" if there is such a thing.