Thinking of heading to Slovenia last week of July, first week of August with the kids (10 and 12).

Background - went there pre kids in Sept 2004 I think and had a fantastic 2 weeks - Soca rafting, summit of Triglav, gorge walks, waterfalls, canyoning trip, Bled, Lake Bohinj,  stayed at Laze and did the tourist caving trips and the Idrija mercury mines. So we're quite familiar with whats available. 

Want to go back again with kids but realise it might be peak season. 

Same happened last year with Corsica, been before out of season for our honeymoon, but went last summer avec les enfants and holiday was marred by everywhere being totally jam packed and over crowded.

Question is, is it going to be the same for Slovenia, and if so, where to avoid and are there some slightly quieter areas to get away from the hoards.

Tick lists would be Via Ferrata (done a few in France with the kids). Rafting, canyoning. Potentially a night in a refugio on Triglav somewhere. (expect these are fully booked all the time).
Maybe some tourist caves in Postojna, and maybe a guided 'wild' trip if they're easy enough to organise.

Will be camping.

Thinking of flying to Venice and driving in, as it seems cheaper than flying to Ljubljana.

Any advice welcomed.

Sam,
  With regards to flying make sure you compare the prices of hire cars at all the airport options as well as the flights. We flew into LJU and out via Venice after a holiday split between the NW part of Slovenia around Kranjska Gora (KG) and Lake Garda. It was cheaper for us to hire the car at LJU rather than the Italian options. We compared all the flight options using the popular skyscanner.net website.

The area around KG gives you access to Triglav and the mountain walks from the pass that links the town and the Soca side of the mountains. Also, there is a good cycle route from KG towards Italy and the Mangrt / Fuisine (spelling?) Lakes which are well worth the visit. Mangrt itself provides some via ferrata options of varying difficulties and the views on a cloudless day are stunning.

Enjoy the trip.

AMH

If you've got the bin-lids with you Sam, Postonja (postojnska jama) show cave is awesome. Travel in on a small railway ride then English speaking guides take you through more of the show cave.  We almost got tired from saying 'oh wow' on the train ride in, with large chambers ever increasing in size and beauty.  We did some haggling too with our BCA cards for a discounted entry (well Edd Williats did, said he knew the queen and shit like that).

As for via ferrata  - Soca Valley up north - some of the old suss lot will probably recall more..... if they read this. 

Cracking country though and friendly people.

Enjoy

Generally speaking you will not find Slovenia overcrowded. Points like Bled, Postojna cave, Portoro? and Piran will be more visited in summer. So you can avoid these. I do not know current situation in upper So?a river valley. It can be full on weekends. But once you are on via-ferrata, there will not be much of crowd around
As you are looking for more adrenaline vacations I advise upper So?a valley. You can combine climbing, cave and rafting in one day. I would not personally, I would take three.
But you know, when time is short...
But if you wan't very lonely wild experience, go to SE to Ko?evje (near my home). You will find Kolpa river with swimming and rafting in clean warm river just bellow most pristine wild forests in these parts of Europe. Not much tourism going on in these parts (apart from Kolpa river on weekends). I can give you contacts to local caving clubs. There are some very nice caves here. And a lot and lot of clean pristine nature. No via-ferrata.
But Slovenia is small, You can get from Bovec in NW So?a valley to Ko?evje in SE in 3 or 4h drive.

Just left Slovenia yesterday. Shame I didn’t see this post before but can give a big thumbs up to all the points in first post. A beautiful country well worth spending time in.
We did the bike ride to the lakes from kranjska gora, via Ferrata near kg, swimming every day in lake Jasna, postojna cave, Triglav from radno polje and dom planika hut. Also swimming in bohinj. Oh and a nice campground in kamnik.
The hike up to the high pasture shepherd’s settlement near kamnik is worth it to see old traditional life. There is a cable car option too.
A word of warning though. They seem to have a big thing against wild camping of any description anywhere be that dossing in your van somewhere or bivvying out. Big fines if they catch you not in a designated place. They do allow camper van parking in some random parking lots like ski lift stations.
Also they charge more than double for parking (car parks) vans over cars for no apparent reason. You must always park in a designated parking spot or you’ll get a ticket or towed - even just unobtrusive parking on the verge in a quiet side road (like we did).
Fuel however is significantly cheaper than Germany so mind when you fuel - we payed a full euro more per litre in Germany.
Oh and you need a carnet/pass to drive their motorways like Austria.