How's this itinerary?

Walk down to the beach in Dubrovnik to find numerous operations with kayak tours. We did a 1/2 day kayak around Dubrovnik and Lokrum - we loved it !

I know this is yet one more single night stay...but consider stopping in the town of Sibenik from Split, then visit Krka the next day. This will save you about two hours of driving back and forth from Zadar. Plus, Sibenik is a nice little town, a highlight for me actually. Give yourself at least an hour or two to explore it if you have time more than just sleeping there. Then spend just a single night in Zadar.

Zadar's old town peninsula doesn't really take that much time to explore, and you can "see" the sea organ + sun salutation in about 10 minutes.

Is it better to spend one more day in Plitvice?

I've read on several forums that it's highly recommended to stay the night before you plan to visit plitvice in the area so you can get to the park first thing in the morning (before tour groups etc). I had asked whether we'd need two nights there was was told a night would be enough.

I think one night at Plitvice will be fine. The key thing is to have some time there outside of day-trippers' hours, when the crowding can be unbelievable.

I believe the longest marked pathway takes considerably less than a full day. I took the second-longest path. I think it was listed at about 4 hours, but I completed it in a bit less than 3, despite being slowed down by the crowding.

The park sells 2-day passes, so if you happen to arrive several hours before the park closes, you could do some walking on your arrival day and finish up the next day.

One day at Plitvice is plenty for most people, I'd think. You can get the gist of the park in a good 5-6 hours or so, really. But it is best to get into the park first thing when they open (7AM in season) to avoid the crowds, if you can.

I've been to Plitvice twice. The first time, I stayed at one of the park hotels, the Plitvice Hotel I think (by Entrance #2 - upper lakes). The park hotels are overpriced for what you get, but they are convenient: you can walk right down into the park and leave your car at the hotel. (If you drive to the park and pay to park at the lot across the highway, you walk past the hotels anyway to get into the park.)

One benefit of staying at a park hotel is that they will stamp your ticket from the day before so you can enter the park again a second day on the same ticket, without having to buy another one. They did this for me; I arrived the first afternoon, entered the park, and then the next morning went back into the park for a few hours before departing, on the same ticket. You might calculate the cost of a two-day park ticket plus two days of parking in the lot, if you drive, vs. the extra cost of staying at an overpriced park hotel. On my second visit, I stayed at a soba (B&B) nearby instead, but I was there only a brief time and had time for only half a day in the park, anyway.