the Black Cave at the end of Postojna Cave
If you don't want to visit the larger well known cave in the busy July and August periods then why not take a bike and cycle through the forest from Lipizzaner Lodge to the Black cave, connected to the Pivka cave and backing onto the end of the Postojnja cave.
In fact there is a "back door" at the end of this cave that leads directly into the Postojna cave and was used by the Partisans to sneak in and blow up the Nazis explosive dump, stored (disgracefully) in one of Europe's natural wonder caves.
More to come..