Been there? I grew up there- I was trying to make it back for Halloween this year but couldn't swing it. BTW- DEFINITELY go there for Halloween some year- it's fucking awesome.
Nearly everything is within walking- er, staggering- distance. Just outside of the French Quarter is the D-Day museum (http://www.ddaymuseum.org/)
, which I have to visit every time I go back. It's been updated with the Pacific Theater of Operations wing, and the weapons display case is always a cool part to see. Right next to it is the Confederate History Museum- a lot of original Civil War stuff, but all the guys who work there are really into the history, so they can tell you everything about particular battles or sequences of events.
If you've got an entire day to kill, then I HIGHLY recommend the Audubon Zoo. It's a streetcar ride and a ten-minute walk from the Quarter. It'll definitely take you all day, but they've got a wide variety of animals from all continents. The Aquarium of the Americas (also in the Quarter) is worth seeing too.
For food, beignets at Cafe Du Monde are mandatory. Plus, they make that good, strong wake-me-the-fuck-up-'cause-I've-been-drinking-for-ten-straight-hours coffee. Cosimo's is another good place for food. Central Grocery's Muffuleta's are also on the must-have list. Also, be sure to check out Southern Candymakers for pralines and other glucosenated goodness.
If you want to search for certain types of restaurants, check out http://www.nolaeats.com/forum/directory.php
For bars, I've always been a fan of Flanagan's pub on 626 St. Philip. Good people, huge TV's, good music, and cheap drinks. Bourbon was never really my scene, but stop by Pat O'Brien's for a hurricane. Those things will fuck you up but good. Also on the mandatory spots- Molly's at the Market for their frozen Irish coffee.
The film industry is booming down there right now, so you never know who you'll run into.