I fish the Slippery Rock creek many times a year. My wife's family actually runs the Heinz camp there so I go there with all my worm dunking family members right up to the damn and fish maybe 8 to 10 times a year. If you get in during the white fly hatch, which I believe is around August, it is like a snowstorm. I have had much success fishing right below the dam stripping a white streamer of either marabou or deer hair. Be careful wading though, it is very "slippery" and you can go from 6 inches of water to 4 feet very quickly. There are many parts of the stream that are stocked with trout, but the best places for smallies are some of the deeper sections of the stream. It can get really low in the summer and you have to know where to locate fish. It is a very nice stream and I've pulled out some 3 plus pound smallies out of there, very fun on the fly.
I also suggest the Neshannock creek. Very beautiful stream with great conservation on the part of the fly shop and local anglers. Not too far up the road from Slippery Rock. They have a fall stocking there too. Check the stocking list on fish.state.pa.us
And lastly the Shenango River. If you search for a restaurant called Danny's in Sharon, Pa and take directions right there, it is a pretty good spot right above the bridge by a small island which is just below the release from the Shenango Dam. You can catch walleye and other warm water fish in the summer time, and can get some pretty decent holdover trout in there too. This is just a little bit farther north than the Neshannock.
You should come back in late October/early November for some Steelhead. Now that's a treat.
Good luck and tight lines!!