Split and North itinerary

Do you plan to rent a car, or will you be using buses and trains?

You have just ten full days, so you won't have time for all of this:

• Trogir (very near Split, easy day-trip, I assume)
• Sibenik
• Zadar (a sort of mini-Dubrovnik without a walkable wall, so not a top priority on a short trip like this)
• Plitvice Lakes National Park. Spend the night before the visit at a hotel inside or very near the park so you can enter at opening time and have some time before the hordes arrive. If you can arrive by mid-afternoon, you can see part of the park later, after a lot of the day-trippers have departed. This strategy is absolutely critical. You do not want to treat Plitvice as a day-trip from Split or Zagreb or as an in-transit visit. This would be a top priority for me.
• Perhaps an island. Others can suggest their favorites. Logistical practicality will be a key issue.
• Istrian Peninsula (but this really needs 4 nights, or perhaps just 3 if you have a car)
• I'd alot a minimum of one full day in Zagreb, more it you're an art-lover because there are lots of interesting museums.

If you prefer to be more adventurous, you could head east out of Split to Mostar and/or Sarajevo.

If you'll have a car, from Plitvice you can drive in an evening to the Istrian peninsula. We spent a full day in Plitvice, so we stayed in Opatija to limit driving that day. The following day we made short visits to Pula and Rovinj on the way to Porec, where we spent the night before continuing our 16 day trip into Piran, Ljubljana and Lake Bled, Slovenia. Then back to Zagreb for our flight.

You wouldn't have time for all that, but could pick and choose.

You don’t say what your interests are or if you are hiring a car. Your first and last days don’t count if you are going to be suffering from jet lag. What interested you to book Croatia?

Split itself needs at least a full day. Nearby Trogir is a half day plus trip. If you want 3 nights on an island, I would take the ferry to Stari Grad on Hvar. Hvar town will be full of noisy partygoers in the summer, so is best avoided, unless you want to join them.

If you hire a car from Split, you could see Krka National Park and Plitvice, but Plitvice will be busy, so stay the night before to try to beat the crowds.

Do you want beach time?

Istria needs at least 4 nights, ideally 5 as it’s quite a drive to get there.

Just updated the post with some more of our interests- thanks for the idea Jennifer- I was pretty vague in my initial post :)

You'd have to be very careful about car size if you need to accommodate four people with "likely 4 bigger suitcases". Trunks of European cars are generally not large at all. Ignore this comment if you've traveled in this same party of four before and know the luggage will fit in the type of car you would rent.

I have no personal experience with theft from cars in Europe, but I believe it's fairly prevalent in places with a lot of tourists passing through (who are perceived as likely to have good stuff in the trunk, whether they do or not). There have been warnings about this in guidebooks going back at least to the 1960s, and that was a time when the typical tourist didn't have a thousand dollars worth of tablets/smartphones with him. Having been pickpocketed in an historic town in Bulgaria (where I'd put the risk a great deal lower than along the Croatian coast), I'd suggest proceeding with caution.

I can't say anything about nightlife possibilities in Zadar (though Google might turn up something) because I arrived fairly late on a rainy evening and went straight to my lodgings, so I only saw the town in the daytime.

If you want to include Plitvice, do the Zagreb return. If Krka scratches that itch, it is close enough for a day trip from Split and I'd go with a Split return.
You could go to Vis for a few days, then spend some time in Split, then stop at Plitvice en route to Zagreb. After four visits to Croatia, Vis remains my favorite for beaches. You do need to use wheels of some sort to get to the best ones though (car can be rented on island, or scooter, or bike if you are of heartier stock than me--steep hill getting out of Vis town and it will be HOT).
If you are going to rent a car, try to limit the bags to carry-on size at least. No car would be necessary though.

To reach Plitvice and to see the park takes many hours. Naturally, you will be tired. I suggest if you are with the car to book accommodation somewhere on D 52 road. Booking.com has a hotel, flight, and car rentals.