Where to go on a Southern US holiday?

My dad and I live in Australia, and we a decided to travel the US over three separate trips. Last year we travelled the west coast, where we saw LA, San Fran, Vegas and San Diego. This year in September we're going on a Southern US trip, and next year an east coast trip. Now I am just wondering, what would be the best places to go to on our southern US trip? The places we have in mind are San Antonio, Dallas, New Orleans, Memphis, Nashville, Orlando and Miami. Ideally, we'd like to see around 4 places, any ideas? Any other cities that would be better to see? What are the good and the bad at these cities?