Richard, I definitely go in the fall or winter: Nothing like your Canadian weather then, but relatively cooler in South Louisiana at that time, though it's rarely cold in New Orleans.
I'd avoid Mardi Gras too: I've never felt the compulsion to be in an episode of COPS or a Girls Gone Wild video !
You'll be tripping over yourself music-wise, LOTS of places from the usual House of Blues to Preservation Hall to everything in between. I'm not much of a clubber these days so maybe someone else can advise the best spots.
For resturants, for big tickets go to Emeril's NOLA or Commanders Palace or of course, Brennan's. My favorite though, is to go to The Central Market near Jackson Square and have the Muffaletta sandwich, better yet, get one and go sit on the Mississippi and watch the boat traffic. The seafood most anywhere is fabulous, and Mew Orleans particular blend of French, Creole, and Cajun is some of my favorite eating on the planet.
J o e y