More Than You Might Have Thought
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There are actually quite a number of wildlife sites as well as CW/Elvis sites besides just the Grand Ole Opry and Graceland more or less on your way. What's even better is that you have the time to enjoy them at a relatively easy pace. Since you mention no interest in cities other than your wife's two targets, I'll highly recommend that you skip I-95 down through Philadelphia, Baltimore and Washington and instead take I-78 west to I-81 south. That would take you near to Gettysburg, Shenandoah National Park, and Great Smoky Mountains National Park on your way to Nashville, as well as set you up for stretches on the scenic but slower Blue Ridge Parkway which runs parallel to I-81. From the Smokies through Nashville to Memphis I-40 is the main road, although US-64 offers a slower and, to me at least, more interesting route along the southern part of the state, particularly between Nashville and Memphis where you can combine a stretch on the Natchez Trace Parkway out of Nashville with US-64 past Shiloh. In Memphis itself, besides Graceland, your wife might want to check out Sun Studios where Elvis got his start along with other legendary singers such as Johnny Cash, Jerry Lee Lewis, and Carl Perkins, as well as Beale Street which houses the current Memphis music scene. You might enjoy Mud Island.
The next leg would have you leaving Memphis on US-78 down through Tupelo (and Elvis' Birthplace) to Birmingham and then US-280 through Auburn to Columbus GA. A couple of short side trips here would be to Warm Springs GA and FDR's Little White House and/or to Andersonville. US-82 would then provide the connection to I-75 south. But rather than just hurtling down the Interstate to Orlando, you'll probably want to make another little side trip to Okefenokee Swamp.
AZBuck