I love these kinds of questions..
I think
Fruition would be perfect, at least as far as food. I've had a very quiet, special dinner there when we had a table by the window, but I've also had a bright, noisy dinner in the other seating area.
Bistro Vendome might be better. They have some excellent fish dishes on their menu. And with Bistro Vendome, you can stroll around under the lights of Larimer Square afterwards. The dining room at night is pretty romantic.