Starting anywhere on Canal Street, walk down to the riverfront, over to Jackson Square, and up to Bourbon Street (which is the heart of the French Quarter). This is probably a good "guided" tour given the time constraints. It doesn't look like much on the map, but there will be enough wonderful distractions along the way that this will easily fill up a morning for a first timer with a camera. Of course Bourbon Street really has to be experienced on an evening or weekend...
Also, because I see that it hasn't been said (and I see that you're in New England!), prepare for the heat -- yes, even in the morning. The midday heat is the worst and it sets in around 9AM. Also, be mindful of walking in and out of air conditioned places. You'll freeze indoors and melt outdoors, and with 110% humidity, you can imagine what this does to cameras and lenses.