I did this in the last month over a weekend. We hiked for 6 hours in Great Falls, MD. We hiked over the park in the trails, and then we visited the vistor's center and saw this great little gate keeper museum there. Then we saw part of the falls. We were going to go to the Billy Goat Trail Section A, but then I saw the entry to Olmstead Island, which I haven't been to at all, and we went there first. It was great, and my daughter really loved the island.
Then we went to hike on the Billy Goat Trail Section A. We got to finish half of it since it got dark. We had to use the emergency trail exit. Then we hiked in the canal towpath in the darkness until we got to the car. It was really nice. :)