If you don't feel like the hype with the little train and light show in the largest cave in Slovenia in Postoina, you should drive a little further in the direction of Koper/Trieste.
If you are satisfied with a day with two at least as spectacular cave tours, choose the cave systems Divaska Jama and Škocjan Jama, which are close together, the latter even being a UNESCO World Heritage Site.
The first cave Divaska Jama at Divača is not that long an attraction, all a bit makeshift (diesel generator as power source) and not designed for the masses like Postoina and Škocjan, but therefore much more "intimate" and charming, with smaller groups (8- 30) and many more detailed explanations. The tour lasts 2 hours, which gives you plenty of time to admire and photograph everything.
The tour descends in a spiral and visits different chambers of the cave. Neither the distance run nor the depth meters are correct at Komoot. It goes up to 150m in depth and up and down here. Then there is no more GPS signal.
The only downside: from the early days of cave discovery and use, ash was transported into the cave to make concrete and fill the floor, which then spread deep into the cave over the years and turned many surfaces gray.
At a very fresh 8°C, we went between enormous stalagmites and stalactites in large halls in different areas of the cave and by 2023 more areas will be released.