There are multiple good day-trips possible from Ljubljana, but I suspect you'd miss out on some nice places in the Julian Alps. But Slovenia is rather small and Ljubljana is actually quite centrally located. It's one of the very best countries in Europe to attempt to see from a single base. And Ljubljana itself is charming. We need input here from someone who has traveled around Slovenia with a car. I've only used public transportation, which limits one's day-trip options.
Take a look at a map and you'll see that Croatia is an entirely different kettle of fish. There is no one point from which you can see all or even most of Croatia. That's a fact. I know Dubrovnik has become very, very touristy (I haven't been there since the 1990s except to change buses). But I wouldn't want to skip it unless I simply didn't have enough time. The drive around the Bay of Kotor in northern Montenegro is also a world-class experience. I agree that Istria is also worthwhile. As already mentioned, full appreciation of Plitvice Lakes National Park (one of Europe's top sights) requires that a night be spent in or near the park. (I've done it as a day-trip as recently as 2015, and it was a very sub-optimal experience.) And Zagreb is quite different from coastal Croatia and--to me--worth more than one day.
I liked Istria a lot, but I think with a rental car you wouldn't need a full week there unless you want some beach or veg-out time.
I do not like rushed trips myself. I always stay longer at each stop than just about any other traveler; I find interesting day trips in order to justify spending (often) four nights in even small cities. Still, I would never recommend that someone spend two weeks in a single location in either Slovenia or Croatia unless that person had a project that was going to significantly limit the time available for sightseeing--remote work, taking language lessons, writing a book, etc. There's just too much to see to be hampered by having to make a bunch of (often long) round-trip drives.
I also wouldn't split my time equally between the two countries, given that Croatia is so much larger and its highlights so much farther apart. This is not to say Slovenia isn't worth two weeks of your time--it probably is, though each of my trips has been shorter than that, but Croatia really needs more time.
I'd plan no more than 10 nights in Slovenia and the rest in Croatia. Before finalizing that division, I'd dig into a comprehensive guidebook to Slovenia (Rick doesn't have enough coverage to be your only resource). You might find you could manage with a bit less than ten days in Slovenia and still see the key sights. Those include at the very least Ljubljana, Lake Bled (beautiful but tiny), the Skocjan or Postojna Caves and the Julian Alps. Vintgar Gorge is nice, and Lake Bohinj is less touristy than Lake Bled.
