If you have never been, you will love Slovenia! We once went to both Slovenia and Croatia and on the way back stayed in Slovenia again because we liked it so much. Have been going back many times ever since. You're in luck because you can go in the low season. Slovenia has become really populair in the last 6-8 years. The last few times that we went in high season, we were kind of disappointed by how busy it was, but that mostly relates to the Triglav area.
The Triglav area (Julian Alpes), on the border with both Italy and Austria, is so stunningly beautiful. Mountains, forests and the most beautiful rivers you have ever seen. It's a great place for people who like the outdoors, hiking, canooing, parasailing, flyfishing and other outdoor sports.
Yes, there are several famous sights, like Bohinj, Bled, Predjama, etc. You will find much info about those online an dthey are worth a visit. Allthough not in high season if you ask me. Ljubljana is lovely. A day or two is enough to see the entire city and you can walk it all.
But Slovenia has so much more! Wine for example! a lot of wine and wine culture. And the salt flats; also very stunning. Good food. Try restaurant Hisa Franko for example, if you like fine dining! Some of our best memories! People are friendly and welcoming.
There are many camp sites, both large and small. Their is a true and honest ecological mindset in the country (willingness to respect and preserve nature).
Piran, as mentioned above is just absolutely gorgeous. Don't skip it! the centre is car-free but there's parking spaces just outside the town. The harbour, the beach, all lovely.
As you might gether from my post, I feel in love with Slovenia and can't stop talking about it, so if you hav eany other or more specific questions (camp sites, etc) don't hesitate to ask.
