Croatia Itinerary 13 nights

It will take you a good two hours to drive to Plitvice from Zagreb; longer depending on where in Zagreb you are staying. So you'll want to get a very early start so you can arrive at the park ahead of the massive crowds that can adversely affect your experience there. Alternatively, I'd consider skipping the first night in Zagreb, driving to Plitvice on your arrival day, and spending your first night there. You won't really have enough time to see Zagreb the way you have things planned.

When I was in Croatia, I spent 2 nights each in Hvar and Korcula. I liked them both very much, but you could skip one of them. But I would not add a night in Split. In fact, 3 nights in Split is a lot, even if one of your days there is a day trip to Mostar. I also think an overnight in Mostar is very worthwhile.

It's hard to give you good advice without knowing what you like to see and do. But my itinerary would be one of these:

OPTION 1
1 night Zagreb
1 night Plitvice
Next day, wake up, visit the park, then drive to Mostar with late arrival
2 nights Mostar
Next day, drive to Trogir or Split
2 nights Trogir or Split
2 nights Hvar
2 nights Korcula
3 nights Dubrovnik (you can still do your day trip to Montenegro, but three nights in Dubrovnik is plenty)

OPTION 2: same as OPTION 1, but skip Hvar (or maybe do it as a stopover on the way to Korcula). Add 2 nights to Zagreb.

OPTION 3: same as OPTION 2, but add 1 night to Zagreb and 1 night to Korcula.

Four nights is a lot for Dubrovnik even with day trips. Although I wasn't crazy about Zagreb, some people to like it; you might consider adding a night there at the front. Read up more about it (there's a recent thread above about Zagreb) to see if you think might like it.

If you skip one of the islands, you could also add an overnight in Montenegro instead of doing a day trip. That would mean keeping the car longer of course. If you wanted to drive to Korcula, you can take the car ferry from Split to Vela Luka on the other side of island (if you were planning to stay in Korcula Town like most people probably do - I did). Then it's a short ferry from Korcula town over to Orebic and then you can drive up the Peljesac - a pretty drive - past Dubrovnik and to Montenegro if you want. Or, detour to Mostar after Korcula and do an overnight there, then to Montenegro and return the car in Dubrovnik, perhaps.

If you are worried about the extra cost of renting the car for a longer period, consider that it might be cheaper per day anyway for a longer rental (short one-way rentals within Croatia can incur drop fees, but longer rentals seem not too in my experience). In other words, it may not really cost much more to keep the car longer. You'd still have a ferry ride from Split to Vela Luka, Korcula. I found driving in Croatia very easy both times I've driven there.

You plan on arriving in Split late in the day, then you only have 3 nights there, which means 2 days, one of which you are spending in Mostar. That leaves you a day to see Split and no time to see the nearby Trogir. I would add a day to Split to cover this.

Much of the accommodation on the islands has a 3 night minimum and so I would opt for either Hvar (I prefer staying in Stari Grad to Hvar town, especially if travelling between June and mid September, which is party season in Hvar town) or opt for Korcula.

Four nights in Dubrovnik could be trimmed by a day.

If you are hiring a car and taking it out of Croatia, ensure that you are covered, which may cost more.