I don't know what you are interested in or when you plan on going. Fall is a beautiful time for this trip and less busy but here are some suggestions. Rte 95 is a bear, there are more stops to see but it can add 3 hrs to the trip. The best route is taking 81 through VA, as it goes right near Lancaster. We lived in PA and now in NC, and spent almost 2 years making the trip back and forth. My daughter still lives in Lancaster.The Sight and Sound theater is a great place to see a show. There are plenty of outlets as well. As Hazel suggested the Biltmore Estate is beautiful but expensive. Asheville itself is very hilly and I believe would be a lot for an elderly person and may be out of your way. Greenville SC is a nice town. She might find MT Airy, NC, aka Mayberry, interesting. It is a small town where Andy Griffth is from. There is a museum, quaint stores and restaurants. It would be on your way North towards 81. The Natural bridge is a quick nature walk off 81. Nice to get out and stretch. Lancaster is with traffic about 2 hrs from Philly. Lots to do there. You can see the Liberty Bell and or just walk around Independence Hall, National Constitutional Ctr and Betsy Ross house. Valley Forge is half way between and has a beautiful park with some sites to see. You can choose to go back 95 and stop in DC and then Richmond.
I would go from Atlanta to Biltmore Estate in Asheville, NC. I would go from Asheville to Richmond, VA. Then I would drive to Lancaster. I like Hersey, PA. I would visit Longwood Gardens in PA. Get a lightweight wheelchair from Walmart, Transport only. That is what I use to travel with.