Croatia (+Montenegro, Bosnia & Herzegovina) - Help review my itinerary?

If you are used to traveling like this, it should be fine. However, with a two-week trip, I like to at least have one or two days where I take a vacation from my vacation where I simply stay in one place and enjoy it without doing much specific sightseeing or traveling. I can get pretty worn out by constantly moving and feeling that I have to see everything.

I agree with Mike W that you're on the go an awful lot. However, all of this is doable if you're okay with that, except I'd recommend you skip Istria given your tight timetable.

A few recommendations:

1. Add an overnight in Mostar. Spend most of Saturday in Sarajevo before departing for Mostar, then spend the evening and Sunday morning in Mostar (so you can have some time there without all the day-trippers).
2. Stay in Trogir on Sunday night (instead of Split). Then you can drive to Split the next morning and have plenty of time to explore Diocletian's Palace before you catch the ferry to Hvar. (BTW, what are you going to do with the car while you're in Hvar? Are you planning on a car ferry? Better to leave it somewhere in Split, I think.)
3. That pushes everything else out a day and if you go directly from Plitvice to Zagreb, you'll have an extra day there; alternatively, add a day in Sarajevo, which I think it deserves -- there's a lot to see there.

If you're not too locked in, another alternative (which will save you some rental car expenses) is to do the itinerary in this order: Dubrovnik, Hvar, Split/Trogir, Mostar, Sarajevo, Plitvice, Zagreb. You can travel by bus and/or ferry from Dubrovnik to Hvar to Split, and then pick up your car in Split.

Istria is really nice. I spent three nights in Rovinj last May, and did a day trip around the hill towns but didn't have the time or energy to go to Pula as well. With only two nights, you'll be truly limited in what you can experience there, and it's a long way to drive each direction. I'd prefer to have more time in other places.

But whatever you decide, you'll have a wonderful time! Enjoy!

Just a few thoughts:

Early May means that the season is only just starting on the islands, as is reflected in the number of ferries running. To go from Split to Hvar island (Stari Grad port) - there are only 3 sailings a day (vs 7 a day in July) so you would be on the 2.30 ferry, arriving at 4.30. This is a car ferry and adding the cost of the car will be c 75 US dollars each way - do you think this is worth it for 2 nights? The morning ferry off is at 8.30 and you will need to be there earlier to buy your ticket and load the car on the ferry.

Having a car is great for exploring but with only 1 full day there, your exploring will be limited.

You can get the catamaran which only takes an hour but this goes to Hvar town and is for foot passengers only. So what will you do with the car? I would be tempted to drop Hvar, much though I love it and spend time in Split and Trogir.

When I hired a car in Croatia last summer, I was not permitted to drive it down to Dubrovnik through Montenegro, so check this out. Some companies do permit this and with others you have to pay more. You will also be paying for a one way car hire drop.