I've been lucky enough to do several trips to Europe over the past few years, using cities as a base while seeing as much nature as I can. Having heard some great things about Slovenia, I took the plunge and booked a trip last June. I was blown away by everything the country has to offer, and I thought I'd share some highlights if you're looking to put together an itinerary.
I'd recommend basing yourself in Ljubljana. I got an apartment across the road from the main bus station and, although it wasn't in the most scenic area of the city, it meant I could roll out of bed early in the morning and get public transport to most of the areas I wanted to visit. They included:
Lake Bled
The bus takes about an hour. The lake itself is stunning and I managed to avoid the crowds by arriving early in the morning, but it inevitably gets busier as the day goes on. Top tip: most people get off at Bled's main bus station, which is about a 10-15 minute walk from the lakefront. I walked around the whole lake and ended up wishing that I'd got off at the next stop - Bled Milino - instead, as it's directly on the waterfront and you get the best views from here, IMO. It also means you don't have to walk around the section of the lake that's on the main road, you can just stick to the much more scenic forest path. I walked up to the castle (without going inside), but I personally thought you have much better views of it from the lake.
Vintgar Gorge
If you head back towards Bled's main bus station after you've visited the lake, there's also a private tour company that runs trips to Vintgar Gorge. There was a lady sitting outside a small shop selling tickets. Not sure if they run during off-season, though - if they don't, it's walkable in around 45 minutes.
The trail around Vingar Gorge is really easy to navigate as you're walking around a boardwalk the whole time (you have to wear a safety helmet) - but getting back to the car park presented more of a problem. I admittedly got lost somewhere after taking a detour to visit St. Catherine's Church, and ended up just Google Mapping my way back to Bled.
Lake Bohinj / Savica Waterfall
Bohinj is around a two-hour bus ride from Ljubljana. The bus stop for the lake is literally right by the viewpoint shown in my first photo. From the bus stop, it's just a short walk to the boat that takes you across the lake, or you can hike if you prefer. I highly recommend taking the Vogel Cable Car (stunning views over the lake and mountains). From the far side of the lake, you can also hike for about an hour to Savica waterfall. Just be prepared for LOTS of steps at the end - they were a killer after a long day of hiking. The waterfall itself is actually behind a gigantic locked gate, and the viewing platform is pretty crowded, so it's hard to get a decent angle for selfies/pics - it is doable, though.
Lake Jasna
Lake Jasna is a 15-minute walk from the main bus stop in Kranjska Gora, an alpine town that's absolutely stunning. I initially stopped here on a tour, but we only had about 20 minutes to snap some pictures of the ibex and power-walk around part of the lake. It wasn't nearly long enough, so I ended up researching buses and coming back to spend a day here. The bus takes two hours each way from Ljubljana, so it's a trek, but trust me, it's worth it. Make sure you duck through the trees on the right side of the lake because there's a gorgeous stream and even more stunning scenery there. They have a restaurant and places where you can stop for refreshments, but it was nowhere near as crowded as Bled and Bohinj, so if you want somewhere to sunbathe and read a book for the day, it's perfect.
Postonja Caves
Postonja was hands down the best cave system I've ever been to. You get a "train" that transports you into the caves, and it feels like you've landed on another planet. There are so many chambers absolutely teeming with weird and wonderful formations, some of which look like they could kill you if they fell on your head. The sheer scale of it is mind-blowing, and they've done the lighting really well. I also visited Škocjan Caves on a tour too, and they were gigantic chambers with daylight flooding in here and there, but not many stalagmites and stalactites - Postonja was far superior, in my opinion.
I managed to get a bus that stopped directly outside the entrance to the caves, but most of them arrive and depart from the town centre, which is a fair walk away. I had some trouble finding the bus "station" on the way back too - it turned out to be in what looked like a car park, but I'm not sure if that was a temporary or permanent arrangement! The shuttle bus to Predjama Castle unfortunately wasn't running, so I missed visiting there - in hindsight, it might have been better to book a tour that takes you to both.
Soca Valley
I took a tour for this one as it involved multiple stops, some of which weren't accessible by public transport. We had a lovely little hike to Kozjak Waterfall and then stopped at various vantage points around the valley. Breathtaking scenery and some of the clearest water I've ever seen. The tour price was definitely worth it.
Piran
I also did a tour to Piran to avoid a lengthy bus journey with changes, but I think it is doable if you change in Koper. Piran is a gorgeous coastal town with a completely different feel - colourful buildings, a pretty marina, and a postcard viewpoint over the town.
Zelenci Nature Reserve
This was beautiful, but so much smaller than I expected - you could literally walk around the boardwalk and up to the viewpoint in a few minutes. You won't need more than half an hour here, and I wouldn't say it's worth the trip in and of itself unless you're going to other places nearby, but I might be biased because I had an absolute mare trying to find it when I got off the bus in the nearest town. What should have been a 20-minute walk took me more like an hour, but I had a lovely - if slightly panic-inducing - detour through the woods. You could combine this with a trip to Kranjska Gora because the bus also stops there on the way back.