I know Slovenia well and just done 2-weeks there in June. PM me if you want any specific details. But whilst there would recommend a few bits in particular:
Lake Bled obviously, just beautiful although does get very busy in the summer. I actually prefer the less touristy Lake Bohinj which is about a 30min drive from Bled. Much bigger and more dramatic looking, with Moutn Triglav overlooking it.
Postjana or Skojan caves are well worth a visit. Postjana you get taken down into the caves on a train, then explore once down there.
As others have said Ljubljana is a great City, such a nice friendly atmosphere, great places to eat along the river, castle on the hill and their beer/bar scene has really improved in recent years. It does get busy but never feels like a tourist trap.
It's a 2-hour drive from Bled, but the Soca Valley region is just stunning, mountainous with the bluest river running through it. Loads of hikes, walks, climbs, rafting you can do here if you're into that.
Piran is a really pretty little town, completely pedestrianised and very Italian feeling. If you do get the time, Izaola next to that is also very nice and Rovinj just down the coast in Croatia is one of the nicest places we've been for years. The seafood and views were incredible.
Don't know anyone who leaves Slovenia without loving it so hope you have a great time! We go most years.