I can speak pretty well to New Orleans even if I have not been now, in about 4 years. For one thing, bless that city's scene, not that much changes; for another, I get a daily e-mail written by one of New Orlean's main food writers, Tom Fitzmorris, so I stay a bit current.
That said, I wonder how I'd spend one afternoon, one afternoon in New Orleans. I assume you are looking for something memorable and, well New Orleans-y, not just, say a good meal on the way to the airport. And there are so many choices, all so correct. New Orleans is as much the corner bar and po boy as it is the centuary old creole tradition. You have to decided, I suppose which is the one experience you want this time. Here's my suggestions:
Late lunch, Galatoires - This is where all the famous dishes still exist and still exist in states that made them famous in the first place. Lunch for two would present an awful choice of which dishes, especially in the appetizer catagory. Shrimps remulade, crab meat maison or crab meat ravigote (which are about the same thing), oysters brochette, oysters rockefeller, you cannot go wrong. Galatoires is famously on the tourist path (hell right on Bourbon), but it is also a most New Orleans of places, and very much a locals place above all else.
Commander's Palace - An alternative if you are looking for fancy. CP is Galatoires just a bit more modern. You can still get shrimps remulade and eggs sardou, but instead of bread pudding, you get bread pudding soufle. Like Galatoires it is as much a tourist place as a locals place. Me, I'd might lean a bit more here than Galatoires for a few reasons. It's a littler warmer/friendlier, and it's a bit more convinient unless you are already in the French Quarter. Still, if you really want to "get" New Orleans, I would go to Galatoires first.
Mother's - There are probably better versions of nearly everything on the menu here at other places around town, and the po boy sammy's are unique in style as compared to other places (say the way Al's would be unique re: italian beef), but no other place gives you as many ideas of New Orleans down-home cooking, done so well, and so accessible. As in the places above, it is hard to pick one or two dishes.
Heading out to the lakefront for seafood, best is probably Brunings. I mention Brunings not so much because it is in the top 5 places in New Orleans (maybe top 25), but the lakefront seafood experience is so different than what you could have in Chicago. Summer is crab season, and if you do not get hard shell crabs in Baltimore, you might want to try here.
Camelia Grill - If you are near Tulane, hungry and just want a great burger here it is, and if your daughter goes to Tulane, she'll be there plenty next year too. It's New Orleans in the sense that a counter-grill place exists with Maitre d' and linen napkins, but it is not really New Orleans food. Just good food.
Let me know if you need more.
Rob