Piggybacking on Matejicek's review, I would like to pipe in as a local. Slovenský raj is part of the Slovenský kras (Slovak karst) area in the East Slovakia and is considered one of the best places to hike as the hikes are interesting: they glide alongst waterfalls and contain loads of iron elements: steps, rails, chains. Some of them are proper ferratas, so you will not be able to get on them without proper equipment (harness and helmet). It is the best place to take your kids hiking and they might not find it that boring. Having said that, in summer season it will be quite full and if you don't wake up early enough, you will be waiting in slowly moving lines... imagine similiar situation in Plitvice lakes but on a steeper hill. Though if you want to go, I would be very careful of my belongings, especially when camping, some of the local residents do have sticky fingers and it's a notorious place as far as petty crime goes. I have done some hikes on my school trip, we stayed in Podlesok, Hrabušice.

Zádielska gorge is also a nice and picturesque hike, though if you don't have your own car, you have to take a bus to Turna nad Bodvou or Zadiel from Kosice. As far as Jánošíkové diery Matejicek has mentioned go, they are a nice hike from Terchová, however there is a similiar problem with many people in summer season: go early or don't bother. The hike then continues on to Velky Rozsutec and can be done in couple of hours.

As far as OUV for this WHS goes, I see no point. The most impressive parts of the Slovak Karst are already included in the Cave WHS shared with Hungarian Aggtelek, that's where the most beauty lies. And the unique carst formations above ground are much more prominent in Vietnam - may it be Ha Long Bay or Trang An National park.