I would recommend Sun Studios, Graceland and the Rock n Soul museum, especially if you like music. Also, you can take a stroll and stop in at places on Beale Street. The brunch at the Peabody Hotel is nice (but expensive). It will probably be pretty hot, so not sure how much time you want to spend outdoors.
My husband and I spent 2 nights in Memphis--we loved it!! Will return to spend more time there. We left to go to Nashville and we were very disappointed in Nashville---Memphis is a much more welcoming city and more active!! Loved the musci in the park and all the things on Beale Street.