One note, I DO NOT want to have to dress up. I like casual.
Since it's December you'll need a jacket, even in New Orleans. I say, put on a sportcoat & chinos, no tie, go to Commander's Palace and be sure to get the bread pudding souffle. At http://www.commanderspalace.com/new_...neral_info.php they say "Dress is upscale, jackets preferred at dinner. No Shorts." You can get away with a lot in New Orleans, including how you dress - except for maybe at Galatoires or a few other places.

Here're some tips I gave to someone who's never visited the city.
http://www.flyertalk.com/forum/showthread.php?t=303249

That time of year you can get Papa Noel hotel rates, and many restaurants have Revillion Dinners throughout December. Windsor Court may offer a good package for a suite & dinner in the Grill Room if you're looking for an extravagently indulgent time.