Castleton is such an interesting area. It marks the most dramatic of the limestone scenery of the area but also the start of the Pennine Way as the geology changes suddenly to millstone grit, adding to the variety of flora. Take a good flower identification book; finding special flowers can become obsessive! The caves are fascinating (in one, there's a trip by boat deep underground; in another there's such a huge entrance that there was a village there with a rope making industry!; in yet others one can explore where blue john stone comes from.) The walking in the area is exceptional; well signed but quite strenuous. There's an atmospheric castle above the town. It's well worth exploring the whole area nearby as the villages and nature reserves each have their own character and interest.