My mom went to Hocking Hills some 50 years ago, and has a picture of her, my uncle, and my grandpa in front of Old Man's Cave to prove it. I always loved looking at the photo, and so I was extremely excited to visit the same area, maybe get our picture taken in the same spot.
On day one, we hiked the rim trail at Conkle's and though it was pretty spectacular. There were several spots where we stopped to take pictures, but none did it justice. It's one of those hikes where you get a sense for how small you really are. The trail was well groomed and easy to follow. Next, we went to the Rock House. There are a handful of sections here where you'll have to climb/descend stairs, so be ready for that. The "house" itself can get pretty dark, so I would recommend you bring a flashlight or headlamp. In fact, one woman slipped inside the cave (there are some puddles/slippery rock), and insisted she broke her arm. Again, the views here were amazing, and the trail was well marked. I would say RH was better than Conkle's, so if you can only do one of those, do the Rock House.
The following day, we went after Old Man's Cave and Cedar Falls. This was a MUCH more rugged trail. You'll have to do a bit of scrambling over large rocks, and through gnarly tree roots. Round trip, the map said it was only a four mile hike, but by the end, our bodies felt like we had just done six. Old Man's is spectacular, but it's extremely busy. The parking lot was overflowing, with people parking on the grass and along the roadway, and may of them were not prepared for the hike (i.e. did not have proper footwear, were physically ill-prepared, or had brought small dogs along).
In the end, we're glad we went, but we're not sure we would go back. It was just far too busy for us, and the image I had in my mind of my mom at the cave could never happen now. The hiking is great, the scenery is fantastic, but there were just too many people.