How can you not visit this largest karst cave in Europe when you are in Slovenia? This cave is well-known across the world because of its stunning "stones" created by nature. The temperature there is approximately 10 celsius degree for the whole year. So it was actually quite warm to me while it was minus degree outside... LOL It takes about 1.5 hours for the whole tour, including 3.7km long journey of a mini train, and walking 1.5km with a tour guide. There were only three tours per day in winter. So make sure you plan everything well before you go. I went for the 3pm afternoon session, and most of the shops and restaurant were closing before the tour started.
I have a tip for you: don't worry if you can't take any good pictures during the mini train journey as it goes too fast. You will still have a lot of chance to take picture when you start your walking session.
When I first saw the picture on the Internet, I thought the cave was too creepy to go to. I thought I would be scared. However, when I was there in person, seeing everything with my own eyes, walking with everyone else, I found it so mysterious and amazing. I was totally overwhelmed by nature, and keep asking myself how this can be possible. It can't be judged by the photos.
I thought it would be very difficult to take pictures of myself as it's quite dark inside. You are not allowed to use flash photography, however, if you try to raise the ISO setting higher, it won't be a problem anymore.
I truly recommend you to visit this cave once in your lifetime, seriously.