On the way back from our trip through four countries in three-day (longer weekend in Austria – we have visited Austria, Slovenia, Croatia and Italy) we have visited Skocjan Caves, and despite being part of a larger group we have enjoyed it. I cannot unfortunately post any pictures here as it was not allowed to take photographs or videos while in the caves, but check their website for some images. Below pictures were taken after we exited the underground tourist path. Second image was taken from the view-point above.
If you will only have a chance, visit the caves – walk over a bridge 45 meters above water is breathtaking – you may get at least an idea of what I am talking about if you will check the link I added above.