Croatia and Slovenia - 3 weeks

Don’t miss Lake Bled, the Julian Alps, and Skocjan Caves.
We had one night in Ljubljana and that was enough. Two at the most.
Where is Dubrovnik in your itinerary?
Hopefully you will be visiting at least one island from Split?

3 nights - Cavtat
2 nights - Korcula
3 nights - Split
2 nights - Zadar
1 night - Plitvice
6 nights - Rovinj (used as a base to visit the Istrian peninsula).
3 nights - Ljubliana
2 nights - Zagreb

What you have looks fine. I presume you are flying into Dubrovnik airport then driving south. I really dont know why you would skip at least 2 nights in Dubrovnik, but, okay.

You said the route is set, but I will still suggest if you want to do something some might find more interesting.

Fly into Podgorica
3 N - Kotor bay area (options: swimming, kyaking, paragliding, speed boats, hiking the mountains),
2 N - Cavtat
2 N - Dubrovnik
1 N - Korcula
3 N - Split (i am guessing you are young so that makes sense, but I suspect the party scene dies down a bit by mid-September)
0 N - Zadar (didnt impress me)
1 N - Plitvice (I think in September this is maybe a wasted night if you plan the trip well)
6 N - Rovinj
3 N - Ljub
2 N - Zagreb

Why Cavtat? Dubrovnik is crowded, but I would still grin and bear it. You could put one of those days toward Korcula and I don’t think you’d regret it given your interests. Looks like a great trip!

If you want a hike near Ljubljana, I’d recommend Šmarna Gora. It’s the classic local hike, not far from the city center, and the summit gives beautiful views over Ljubljana and the surrounding Alps. It’s an easy half-day outing and perfect if you want to get outside without traveling far.

It looks real good to me! I do question the Cavtat vs Dubrovnik decision a bit. I liked Cavtat, but it is a small town with not much to do other than hang out and beach. I’m assuming you’re planning on day trips to Dubrovnik in there. Perhaps counterintuitively, you’re now committed to Dubrovnik sightseeing at its worst, most crowded and hot times alongside all those staying in Lapad and cruise ship folks. If you stay in Dubrovnik, you can enjoy the city in the mornings and evenings, and day trip or hit the beach out of town to avoid the crowds. And for all the challenges that come with negotiating one of Europe’s tourist hot spots, I found Dubrovnik an easy place to enjoy.