I would second everything @5foot5had already said. I absolutely loved Slovenia - DH and I had a couple of weeks there last summer.
Lake Bled is beautiful - row out to the island, lovely walk all round, swimming at the lido and from the banks at the rowing club end.
Bohinj is also beautiful. Lovely walking both by the lake and up in the mountains.
Postojna caves and Skocjan caves are really interesting (I preferred Skocjan as less touristy).
The Vrsic Pass was great. Beautiful views and the Russian Chapel is worth stopping for a look. There is a very interesting museum (displays also in English) in Kobarid about WWII, which we popped into for a quick look and ended up staying for ages (neither of us are particularly history buffs!).
Piran is a lovely seaside town that was part of the Venetian empire.
Ljubljana is a great small city - castle, lots of cafes and people watching opportunities.
There was a great mix of people from all over Europe - lots of different languages being spoken - but that also meant that everyone’s second language was English so we had no problem talking to people or understanding what was going on. Really friendly atmosphere. I hadn’t realised that the NW parts of the country have at various parts of its history been under Italian control, which means there’s a big influence on the food.
We hired a car and driving was really very straightforward. However, we took a bus out to the airport at the end and that was dead easy. While we were at the bus station it looked like there were plenty of bus options to get around.
I think you’ll love it!