Plitvice to Istria

I was there two years ago and I would say If you want to see the park thoroughly half a day is not enough. I had to make two hiking tours, one with small train to the starting point, then hiking to one part of the lake. The other part was in a boat to another part of the lake where you can hike to exit point from where you can get back to start point by another small train. Maybe this can be done in couple hours in one day if you are in really good condition but I wouldn't drive after that without a good rest. I also drove to Rovinj after Plitvice and it is not a long drive, from Plitvice to Zagreb-Rijeka motorway (via scenic road through Lika) and then to Rijeka and further to Istria.

Your basic plan for Plitvice sounds good. Stop in the little shop near the entrance and buy a trail map so you can wander at will. I opted to take the second-longest pre-mapped hike in 2015. I think it was estimated at 4 hours (I may be misremembering). I started out in late morning (don't do that!), the park was mobbed, and I had a very, very gimpy ankle to worry about, so I was not moving rapidly at all. (On the other hand I take very few photos.) I still finished that hike way earlier than expected; I think it took less than 3 hours. This is by way of saying that if you opt for the longest hike (I think they said about 6 hours), you might well finish in not much over 4 hours if you don't spend significant time eating for photographing the park. That hike doesn't take you along absolutely every path in the park, but it could well be enough for you.

Someone else here reported having a pleasant visit to the park in the late afternoon, so you might consider buying a two-day pass and doing part of your hiking on your arrival day. The entrance fee in September is HRK 110 for one day (about $16) or HRK 180 for two days (about $26). If it makes you feel better, HRK 180 is what you'd pay for just one day in peak season.

I visited Plitvice on a day trip from Zagreb (don't do that!) and I didn't drive at all in Croatia, so I checked your propsed drive on ViaMichelin. It estimates the time required--possibly optimistically--at just over 4 hours. My guess is that you can manage the drive to Rovinj after visiting the park if you get an early start (which you really, really need to do because of the crowding, anyway), even if you don't see any of the park the night before. On the other hand, it would be a shame to feel really rushed if there are additional trails you want to explore. And all bets are off if one of you is a serious photographer. The other issue is weather. Odds are very good that it will be really nice (if not actually hot), but you might be slowed down by rain.

The driving route takes you near Rijeka and Opatija, either of which would offer lots of lodging opportunities. Rijeka's about a 3-hour drive (optimistically?) from Plitvice Lakes; Opatija would be just a tad farther. If you chose to stay in one of those cities, you could see Motovun and Groznjan the next day before heading to Rovinj IF leaving your belongs in the car wasn't too risky. (Honestly, I just don't know; you'd for sure want to have an enclosed trunk for this ploy.) That would free up your time in Rovinj for that town, Porec, Pula, Vrsar and any other coastal towns of interest.

If you leave Plitvice mid-to-late afternoon, the drive to Rovinj isn't bad on the expressways. The expressway ends after Rejika, as you enter Istria, but the two-lane road from there to Rovinj is good. I took the coastal roads from Plitvice to Rovinj and still made the drive in a reasonable amount of time.

We made the drive from Rovinj to Plitvice one afternoon and it is definitely doable without making a stop.

We arrived into the Plitvice parking lot by 7:30 am and hiked through all of the paths, stopped for numerous photo ops, and were on the road by 2:30.

Thanks so much for the suggestions and input. It is greatly appreciated!

Driving the Autoput to Istria is easy. After crossing above Rijeka and Opatija (do not stop for either, at least on your way west) you come out of the Mount Ucka tunnel and follow the traffic SW to the Y junction at Kanfanar and follow signs off the tollroad - yes, you pay for the freeway- to Rovinj. Do not carry on south to Pula. Parking in Rovinj in September is easier than August - pull in to the huge city lot on the right after taking a ticket at the automated gate. If your accomodations have other parking options, use those. While there are many reasons to drive north to Groznjan and Motovun or other inland villages, you can spend a good day hiking the coast path to the south of Rovinj starting at the Zlatni Rt/Golden Cape park with its climbing routes in the quarry that built Venice. In September the crowds have left. If you are going to Slovenia anyway, you could see Motovun and Groznjan on your way north after leaving Rovinj: coffee in one, lunch in another . If you have to leave Rovinj, that is. Find Cafe XXL in a park below the church at the top, and reward yourself with a cappuccino or a beer and watch the sea. Hiking around Rovinj harbour to the north, past the boatyard and old railway station, will give you a stunning skyline view of the old town. As will taking the hotel boats over to Hotel Katarina or Red Island. You pay on the way back. In Rovinj, the rule is, always stop for sunsets - and moonrises.
When driving in Croatia, my friends recommend the yellow Tifon gas stations - new and good fuel. You can save the east coast of Istria for another year. You will be back.