We travelled as a couple through Slovenia, Croatia and Bosnia last summer.
One thing to think about is your travelling style. Are you going to be on the move every night, or every other night? Or are you going to slow down and get to know the (fewer) places you visit better? Having done both, I find myself leaning more and more to the latter option.
Lake Bled is beautiful, and worth a full day. Flying in to Ljubljana, taking a taxi there (our flight was late and we missed the bus. But the taxi, though more expensive, was not catastrophically so), spending an entire day exploring/visiting the castle/walking around the lake/taking the boat to the island worked well for us.
We rented a car, and planned to take the Vrsic pass down into Kobarid. But we went in early May, and were warned at the start of the pass that it wasn't safe for our little rental car. We wound up detouring through Italy. Seeing the rusted, abandoned border checkpoints as you crossed the border and back was interesting.
We stayed a night in a small tourist farm way up in the hills outside of Kobarid. Very memorable. Kobarid has a great World War I museum that's definitely worth a morning, and there are forts, caves and other leftovers from the WWI front all over the area.
We stayed in Piran. (Warning, if traveling there, avoid staying at Max's, as he might decide to shut his place down and leave you abandoned without notice). Piran is a very enjoyable town, worth staying a full day, or maybe even two.
We visited one of the caves on the way back to Ljubljana. Worth doing, especially if you have never seen a large cave before. Dropped the rental car in Ljubljana.
Then spent a couple days in Ljubljana. Rented a student's flat in the old center. Ljubljana is a very enjoyable city. Interesting things to do and see. Very pedestrian friendly. I wished we had had significantly more time to spend there.
We enjoyed Croatia as well, and found Mostar and Sarajevo in Bosnia fascinating. But if you have never been, the nearby place I think I would add to your visit is Venice. Venice is unique.
So far we have travelled to Italy twice, Britain, Scotland, France (both Paris and south France), Portugal, Prague, and most recently Slovenia--Croatia--Bosnia. Some trips as a couple, some bringing a number of teenage girls.
Of all these places, I would say Slovenia was the easiest place to visit. Very clean, very cultured, everyone speaks English, everyone quite friendly, great food, inexpensive, great and varied scenery.
If I were to live in Europe, I think I would choose Slovenia.
Now to figure out the next trip. Denmark, perhaps?