We did this trip a few years ago, albeit in May.
We stayed two nights in Lake Bled--the night we flew in from the States, and the following night. The one day we had was enough to visit the castle, walk around the lake, take the boat out to the island, and have a nice dinner. An additional night might have allowed us to visit some interesting things near, but not in, Lake Bled.
We wanted to drive through the Vrsic pass, but regrettably it was still snowed in, so we detoured through Italy and went over a different pass. The two most interesting things on that detour trip--seeing the abandoned border control stations at the point where we crossed into Italy and then back again. And visiting an old and not particularly kept up monument commemorating the battle that took place when Napoleon brought his army through the pass to conquer Croatia.
After that detour, we stayed at an agriturismo in the hills above Kobarid. That was interesting, and very different from most of the other places we stayed on that trip.
Kobarid (Caporetto), as the scene of the front line fighting between Italy and Austro-Hungary in WWI, has a very interesting (to me at least) museum devoted to that war, and also a fascist style monument to the Italian war dead built by Mussolini in the interwar years. Those two things are worth a half a day.
If you could stay a second night, I would stop somewhere else along the Soca river, and take the trip up to the top of Slovenia's largest mountain. (Because it was still snowy in May, we didn't really have that option).
We traveled on again, stopping at an Istrian hill town (wonderful), and staying a night in Piran, on the Adriatic coast, before returning to Ljubjana, stopping at one of the caves on the way back. After returning our first rental car, we then stayed several nights in Ljublana, before taking the train to Croatia, where we picked up the second car, and headed to the Plitvice lakes and points south.
I really liked Slovenia. Its a very civilized country, and just about everyone speaks English (its the mandatory second language in their schools, as I understand it). If I were ever to move to Europe, that's likely where I would move to.