Great hiking here. We did the 5.3 mile loop starting on the Lower Gorge Trail to the Red Pine Trail (Upper Gorge Trail was closed), and then back to where we started using the Rim Trail. Great views of Lucifer Falls along the Rim Trail. Unfortunately the swimming area next to Enfield Falls was closed. (We were there June 13, it opens to the public on June 22).
Viewed from the Canadian side on my most recent trip (June 2014). It's well worthwhile to make a dinner reservation at the Skylon Tower. The view of the falls from there is awesome.
Pretty cool castle / mansion, and very well restored and preserved. The tour was definitely worthwhile. No flash photography allowed inside, but most rooms have plenty of natural light coming into them.
The steps down to the base of Webster's Falls was closed when I visited (June 11, 2014), but it was still a great view from the top viewing area. There's a nice trail there that you can walk a short distance to view Tew's Falls as well.