Do you normally travel like that, staying only one or two nights in each place? You are allowing so little time to see the places where you are stopping. Ljubljana is a really charming city; you'll have a few hours to sleepwalk through it, then you're getting up the next morning and going to Bled. My bet is that you won't get an early start (jetlag), then you have to check out of your Ljubljana hotel, get the bus or train or a taxi (might be reasonably priced for 4) to Bled and check into your second hotel. You'll have a few hours in Bled and/or at the Vintgar Gorge but no time to go farther. Then it's Day 3 and you're on the move again.
I'd consolidate the Ljubljana and Bled stops. Stay in Ljubljana. It's easy enough to day-trip to Bled, and you'll save some time by not checking into two hotels. You have to go back through Ljubljana anyway when you're ready to head to Zagreb. You really need at least 3 nights for Ljubljana and Bled when they are the first destinations.
Zagreb is OK with two nights if you don't plan to do much in the way of museums (it has a lot).
Plitvice is OK at one night as long as it's the night before your park visit. Otherwise, you'll be arriving from Zagreb at the same time as all the day-trippers and the park will be a madhouse. (I made that mistake in 2015--don't do it!)
Zadar is OK at two nights. I really like Zadar, but I'd axe it from your itineraries and add the time to Ljubljana (Itin. 1) or Rovinj (Itin. 2).
I would not go to Istria for just two nights. It's a lovely area with at least four towns worth seeing (Rovinj, Porec, Motovun, and Groznjan). The cute coastal hilltop village of Vrsar was nice when I visited because there were no other tourists visible (it may have been a freaky day), and there are some Roman ruins in Pula (though no one here seems impressed by Pula). As of 2015 there was a zip line in Pazin.
It typically takes longer than expected to get to and from islands (getting to the port, waiting for the ferry, etc.). I would consolidate Hvar and Korcula into one stop.
If you haven't already done so, check your flight options. It may not be easy to find reasonable travel times unless you're departing from a major market.
Speaking of "market", both Zagreb and Ljubljana have markets of various types. The crafty sorts of things usually happen on the weekend, with the food market occurring multiple days a week. Some quick Googling will probably pin down the information you need to be sure you don't miss something going on while you'll be in town.
