Hiking in Pennsylvania isn't particularly known for it's danger, but it can be dangerous due to seasonal changes, and unexpected weather! Also, some trails are pretty strenuous, but for the most part if you're experienced at all, you won't have too hard a time.
But Glen Onoko Falls can be an exception to that. It covers rumored "haunted" ground--the story differs, but basically an Indian princess named Onoko may have thrown herself over the falls when she was told she couldn't be with her love.
The real danger lies in the 5.9 mile Glen Onoko Falls Trail, which is a very intense loop hike with 875 foot rise.
The trail can be steep and treacherous at times, so (as always!) you should be wearing proper footwear. The trail is unmaintained with inconsistent blazing and lots of erosion. At times you will be walking on stone steps, remnants of stairways built into the mountain at the turn of the 20th century by a hotel/resort which was destroyed by fire in 1911! Pretty amazing, but it can add up to be a dangerous situation.
Most hikers recommend taking the switchback loop trail because coming back down the falls trail would be more challenging than going up. The trail is marked but not extremely well. We asked advice from fellow hikers several times before finding it. You follow the falls trail (orange blazes, arrows, etc) until you stop seeing water. Shortly thereafter, you'll see a large boulder formation directly in front of you. To the right, there are white blazes that mark the trail that loops back to the Glen Onoko parking area. It takes about 2 hours to do the 2.1 miles! Enjoy!