For a perfect date on a perfect winter afternoon, I suggest skating at the Vieux Port. Bundle up because the wind off the river can be cold.
The scenery at this spot is amazing, and the ice is well-maintained. At $6 per adult, it was an affordable afternoon's entertainment...although the snacks and hot chocolate are overpriced, even in comparison with the Rideau Canal in Ottawa. Skate rental is cash only, so be sure to have some money in your pocket if you don't bring your own skates.
Each day has its own musical theme -- Mondays classical, Tuesdays world music, etc.
I imagine it would be a wonderful place from which to watch the Saturday evening fireworks at the Vieux Port (or New Year's Eve).
We had a really nice time and will definitely go back. (If you agree, a seasons pass is $28, I think).