What's your best date night in Jax? Mine was to take the metro through downtown at sunset so we could go over the Acosta bridge when the blue lights were on and then head into San Marco for dinner and drinks at one of the cool little restaurants like Cafe Carmon (which is sadly not there anymore). Afterwards nothing beats cruising the River Walk or the beach.
Certainly can't go wrong with the upstairs deck at San Sebastian! My wife and I have had more than one date-night end up there. I'll keep it in St Augustine with my recommendation of getting one of the tiki huts at the Conch House and watching the boats cruise in and out of the marina while enjoying a great meal and a cocktail or six.
Most anywhere my Bride and I can go without the kids.Clarks Fish Camp, Ko Ko's, Outback and then wherever whatever. I'm going to give San Sabastian Winery a try one evening, might even try to set it up with a bed and breakfast
Ragtime in Atlantic Beach is quite nice. The Atlantic Beach "Town Center" area is in general one of the nicest places to go in Jacksonville. You can eat at a variety of places, drink at a variety of places, and go down to the beach. Or drink and goto the beach at Lemon Bar.
Casa Marina is also a very nice restaurant which offers second story ocean-view seating.