I like Zagreb, it sort of speaks to my relaxed tourism; especially Upper Town. But you said you were going for nature, so I guess I would suggest spending one night, then the better part of the next day before heading on to your humdrum accommodation at Plitvice. Do Plitvice early the next day before the crush hits and then move on again; I guess Krka Park. Sorry guys, Plitvice in all its glory didn’t light my fire.
You said for nature, but then you have 3 nights in Split. Maybe someone can recommend a more palatable village not far from Split and the Blue cave and then you do both as day trips? I guess some of it had to do with the season and the day of the week. Part of what turns me off in Split is that it has become a drunk European Trust Fund Brat hang out. But that is Seasonal and Weekends.
If you want nature, move quickly down the coast and do some time in Montenegro. No place more beautiful, fewer tourists and less expensive.
This is the sort of trip I would get some planning advice on. With your interests, with a little inside knowledge of memorable locations to stay in the country and good transportation options it could be a pretty special trip in the most beautiful parts of the country with day excursions into the tourist traps. This is the lady I have used a few times in Croatia and think she did an excellent job on the last 4 trips she planned Dijana Krkotic, Doclea Travel
+382 69 277 749 (phone or WhatsApp), [email protected], www.docleatravel.com But there are some others listed in other posts, so hunt around.