In reply to samt:
I went with my partner in July 2016 and we had an amazing time. We flew in to Ljubljana and hired a car. Spent a few nights at Camping Zlatorog on Lake Bohinj (good walking around there, and the lake is stunning. Only a few miles from Lake Bled, which will be jam-packed that time of year). I imagine the campsite will be heaving too, as its relatively cheap (but not actually that cheap!!) and in a great location. It was pretty busy, even in early July!! Nice temperature though.
We then went up to Camp Vodenca and did a bit of kayaking (Oven Kayak - highly recommended). Campsite was excellent there, and cheaper than Bohinj, but much quieter. She told us it is usually very busy in the holiday season. its within walking distance of the town too. I think there was lots of mountain biking round there too.
Then I think we visited the Skocjan Caves, after that which was incredible - highly recommended, despite it being expensive, it was really good. I'd recommend the 'extended' tour as well.
Then we stayed on a lovely little campsite (in the garden of a vineyard) in camping Vrhpolje. I believe there is climbing on the cliffs around there, although we just went for a couple of relaxing nights. It was probably one of the best 'campsites' ive ever stayed in - 5 large pitches, wine on arrival, suitable for kids and next door to a great pizza place.
I think then we went to Idrija to the mercury mine there, which was very quiet, but the tour was cool - proper deep in the mine, and all in English.
Then we spent a few days in Piran, which was very very hot, very beautiful and again highly recommended for a relaxing last couple of days.
I would love to go back and do some VF (although i'd imagine better is available in Dolomites), climbing and more walking... 2 weeks is not enough, but certainly got a flavour. If you're competent paddlers, the kayaking on the Soca was incredible, probably one of the best organised activities i've ever done (we did grade 3 roughly, but you can go as extreme or as civilised as you want).
My parents did a week or so there in the middle of the summer holidays, staying in a villa near lake bled and said it was busy and very very hot, but they loved it too.