That's great that an FHU site was available. When we go to Geneva SP in OH, they have just 4 FHU in the campground, but only 3 are available because one's occupied by the campground host. Your chances of making a reservation after Memorial Day for one of the remaining FHU sites is slim to none; especially for a weekend.
My parents would tell this story about the time they made a day trip to Swallow Falls to see the waterfalls. However, there was nothing but a trickle of water flowing over the falls due to an extended dry spell. Let's hope there's been normal weather when you make your visit.
If you've never been there, you might want to try Ohiopyle SP, located about 35-40 north of Swallow Falls in PA. The Youghiogheny River flows through the park with its waterfalls and rapids. The Great Allegheny Passage bike trail is popular and has impressive views of the river gorge from its bridges. Ohiopyle is the third largest state park in PA.