I think my favorite is probably Jiko at Animal Kingdom Lodge. It's easy to make a whole night out of it. The resort is beautiful with tons to explore in the lobby, the gift shop and the savanna (not sure if they still do but they used to offer night vision goggles after dark in some areas). Victoria Falls Lounge is one of my favorite resort lounges for a pre or post dinner drink and then Jiko itself is just amazing -- food, wine, and service.
Agree with everything jennyq87 stated above about Flying Fish -- another great choice. Although both it and Jiko are just reopening so I'm basing this off past experience and really crossing my fingers that it continues to hold true.
Topolino's is another choice -- that one I have been to recently. The whole resort is beautiful, gorgeous rooftop deck off the restaurant with a great fireworks view (they do not pipe in the music but you can step out to view) and the food is delicious!
Not that any of these are formal but for an even more casual date night, I think anything in World Showcase followed by a stroll around the World is fun.
