Just back from our Slovenia trip, so thought I'd add my itinerary to the thread.
We did 3 nights at Bohinj (arrived late on night 1, so 2 full days) and loved it. Walked around the lake, inc the trip up the gondola where we had a walk and lunch, on day 1, then paddle boarded, walked to a nearby village and relaxed by the lake on day 2.
The next day we got the bus to Bled (40 mins) where we stayed 2 nights. In Bled we rowed to the island (obvs!) and mooched around on day 1, then did the castle, the walk around the lake and an afternoon at the Lido (inc lake swimming) on day 2.
Whilst the 2 lakes are close together, I'm gald we overnighted at both, rather than visiting one from the other. It was a very different vibe at each (Bohinj much less touristy) and they're really quite different.
From Bled we got the bus (1hr10 ish) to Ljubliana. As PPs have said, it's a small city. We had 3 nights, and I wondered initially how we would fill the time. Easily - as it turns out! Afternoon at the castle on day 1, long walk around the river banks on day 2 (stopping for refreshments along the way...), then another walk in Tivoli on day 3 (the hilly area behind the gardens is massive), plus mooching through the narrow streets/squares and then back up castle hill for a drink at the wine bar there. We also did a cookery course and learnt to make traditional dumplings. So we easily filled our time and didn't go in any of the museums/galleries, or do a boat trip etc. I was glad we had the time.
From there we got the bus to Piran. Again for 3 nights - we had one day at a beachclub in Portoroz (2 mile walk along the sea front to get there), then 1 day walking down the other side of the peninsula to the salt pans. There are really good circular walks mapped out and clearly marked along the route, which is helpful. Plus exploring the town, visiting the church/bell tower, swimming etc.
The only thing I'd love to have done but didn't was visit the caves - not the ones beginning with P, but the ones starting with S. Apparently they are way better and much less theme-park like. If we'd had a car, we definitely would have gone, but we had decided to travel by bus, and the excursions were all at inconvenient times for us and/or very expensive indeed.
It was a lovely country to visit though. I'd go back for sure - I feel we just dipped a toe in the water. We were pleased we decided to stay in 4 locations, and having 3 nights at each (apart from Bled) meant we didn't feel like we were constantly unpacking and repacking.
The people were lovely, everything was so clean, the wine was tasty, the food too, and the views/sights were interesting. Weather was excellent too. I have brought back an unopened packing cube of long sleeve tops. I'd definitely recommend it.