MAURICE K GODDARD STATE PARK
It's in Mercer County. If you like fishing, this place is basically heaven because of Lake Wilhelm, a massive 1,860-acre lake packed with largemouth bass, walleye, and monster muskellunge. Grab a boat. You can launch anything up to 20 horsepower here, which means you can cruise around the water all day without dealing with crazy, roaring speedboats. It's actually pretty peaceful.
Maybe you hate boats. That's fine. You can hop on your bike instead and tackle the John C. Oliver Multi-Use Trail, a paved 12.2-mile loop that hugs the shoreline and gives you some killer views of the water. Keep your eyes peeled for bald eagles. Seriously. They nest near the lake, alongside osprey and tons of migrating waterfowl that drop in during the spring and fall. So, pack some snacks and go explore.
There are no entrance fees or day use fees for any state park in Pennsylvania.