The Kildoo Trail in McConnell's Mill State Park, PA, is an easy 1.3 mile one-way hike along the Slippery Rock Creek. This trail has various small waterfalls leading down to the creek and plenty of massive boulders to sit on and watch the river flow by. There's at least one sizeable waterfall along the trail that sits about 10 feet above its creek too.
The trail starts at the McConnell's mill/covered bridge and ends at Eckert Bridge Rd. 1.3 miles one-way.