Northern Croatia

First, can you clarify the following:
1. How many total days do you have on the ground?
2. Where are you planning on starting your trip (flying into)? Ending your trip (flying out of)?

Five to seven days. Starting in Zagreb. THanks for
your interest. Look forward to your reply
Steven

The country is quite expansive lengthwise, so the bad news is that you're trying to squeeze in a trip that would take much, much longer than 5-7 days (and that's if you're going by car, which isn't clear from your posting). Think about it...even if you were to spend a minimum of 2 days each in Split, Hvar, and Dubrovnik, that's already 6 days accounted for. And that doesn't include all the "northern parts" you listed above Split. If I were you, I would focus on one part of Croatia only...north or south. You don't have enough time to go from the extreme north to the extreme south (it's more than a 6 hour drive at least from Zagreb to Dubrovnik) while hitting so many places in between (you have to factor in transport time and then time at each point of interest).

Do you have a guidebook of Croatia? I used the Rough Guide and it was very helpful in planning.

Five to seven days is barely enough time to see Zagreb to Split via Rab etc, let alone seeing Split-Hvar-Dubrovnik, which is another week's trip at a rush!

If you have 5 days (this means 4 nights!) - 2 nights in Split seeing Split and Trogir then bus to Dubrovnik and another 2 night there. For 7 days i.e. 6 nights, visit Hvar or Korcula for 2 nights. Note many apartments on the islands have a 3 night minimum booking. Note that a couple of days this week, the catamaran between Split- islands- Dubrovnik hasn't been running due to the weather - this is also a risk in mid May. Your short timescale doesn't allow much wriggle room for difficulties getting off the islands.

I am reading this as you have 5-7 days to travel between Zagreb and Split, and you are looking to fill that time. Assuming you have the Split, Hvar, Dubrovnik bit planned separately, that is plenty of time to explore some (but not all) of the places on your list. Just read about each and decide how many days you would like for each. A day or two for both Zagreb and Plitvice and you are halfway there.

7 days:

2 days Zagreb - drive to Plitvice Lakes and stay
1 night - tour park starting early and drive late to
3 nights Split - side trip to Trogir

I'd leave off Dubrovnik, unfortunately. Only if you could add 2 nights would I go there

Thanks to those who have offered ideas based on my ambiguous information.
We are booked with Country Walkers to do a walking trip starting in Splt and ending in
Dubrovnic over a period of 10 days. We are interested in seeing the northern portion of
the country on our own by car, starting in Zagreb. We can give this another 6-7 days.

At the moment, I am planning to leave Zagreb in the morning and drive to Opatija
and spend 1 1/2 days in and around that town. The second morning we will drive to
Plitvice and spend the day there. (This leg seems to be about 2.5 hours driving).
We will leave the park in the evening and drive (1.5 hours) to Zadar. We will spend
2-2/1/2 days in and around Zadar, going perhaps to Pag town, Krka, or Sibenek.
The last days we will drive to Split, arriving in the late afternoon, joining our tour
group the next day.

I urge you to alter your plans for Plitvice so that you arrive late in the day and visit the park first thing in the morning. I fear that even in May, there is a risk of a very heavy visitor load. It's much more pleasant to see the park when the walkways are not totally jammed. Many people make day-trips to the park; you don't want to be on the same schedule they are.

Good suggestion. We are now going to drive from Zagreb to the Opatija area where we will spend the
day. That evening we will drive to a hotel near the park . Any recommendations for places to stay near
Plitvice? Any experience with car rentals from Zagreb? Hotels in Zadar?
THanks
Steven

I have never stayed near the park; that wasn't necessary on my earlier trips, so I only learned of the current crowding the last time around (I am now The Voice of Experience). You can find some listings in or near the park on booking.com, along with reviews. There is at least one largish hotel inside the park. You know you'll be paying a premium for that, but it may not be onerous, especially in May.

Zadar's very pretty (and has good window-shopping possibilities), but do check the location of the lodgings you are considering. The old town is on a peninsula, and some hotels/apartments might leave you with a sort of long walk from the historic district. Of course, choices may be somewhat limited if you need parking.

Edited to add: I hope you'll allow yourselves some time in Zagreb. I've only been through Opatija once or twice on a bus, but while it seems a pleasant place (old-timey coastal resort vibe), it's my impression that Zagreb has a lot more to offer and is visually more interesting for just wandering around.

We stayed at Hotel Degenija by Plitvice and ate a great meal at their restaurant.

We rented our car from Sixt in Zagreb and dropped it in Dubrovnik. They were cheap and easy to work with. We normally rent with Hertz or Budget but I would definitely rent from them again.