The Memphis Zoo is quite nice and they have added a couple really nice additions since I visited (the hippo exhibit and the grizzly / North America complex). I loved it largely because I am a cat fanatic and they have a nice cat loop, with something like ten species all on a circular path. I flew into Memphis and stayed downtown and took the bus to the zoo, so that is easily doable if you won't have a car. If you have a car then obviously you can get there and the other places mentioned above (which I have never heard of until now).