I would not visit Plitvice Lakes on a day trip from anywhere. It's critical to have time in the park before the day-trippers arrive and/or after they have left. Otherwise, you'll be marching around those walkways (without handrails, by the way) in lockstep with thousands of others. The park is gorgeous, but you need to have one night there to be sure of enjoying it fully.
Rovinj is a place your parents could stay longer and add daytrips. Rovinj is very atmospheric but mostly monochromatic. Porec is very colorful (and roughly equally touristy). Pula (which I haven't visited) has Roman ruins. The little hilltop village of Vrsar is worth a brief stop. I suspect parking will be challenging at Rovinj lodgings that are close to the old town. To make things a bit more relaxing and avoid having to deal with the car on some days, they could consider taking a bus to Vrsar/Porec and/or to Pula. If they did such side-trips early in the Rovinj stay, they could hold off on picking up the car till later. I'd use the car for Piran and the interior (Motovun/Groznjan).
Note: I haven't verified that rental cars are available in Rovinj.
ViaMichelin says the drive from Rovinj to Piran would take about 75 minutes, but there's a Schengen border on the way, so that may be optimistic. Still, it seems Piran could be a daytrip from Rovinj if the bus schedule from Ljubljana to Rovinj works for them. I used that bus in 2015 but don't remember the timing. I have not bussed between Rovinj and Piran; I visited those towns on two separate trips. I'm not saying it is definitely better to drop the separate stop in Piran, just that I think it may be a viable alternative to reduce the number of hotel changes.
I like Montenegro a lot, and Kotor is very picturesque, preferably on a day when there is not a monster cruise ship in port. (This can be checked on the Internet, though I don't know that the information is 100% accurate.) One of the regular posters here has mentioned that he found Kotor quite similar to Dubrovnik. I suspect that's a fair comment; I did not visit the two towns on the same trip. There are a lot of other interesting places in Montenegro as well as some very nice countryside. If there's only about a day and a half available, I might be tempted to drop the Montenegro visit and add the time to Croatia. I would give this really serious consideration if it appeared that my Kotor day was going to be marred by thousands of daytrippers off a mega-ship. I had both types of days in Kotor, and there was a huge difference in the atmosphere.
Assuming the Kotor stop lives on: It sounds as if you're planning for another car rental for the trip from Dubrovnik to Kotor. This can be done by public bus or probably by bus tour, but if additional stops are desired, a car would be nice to have. Be sure the rental company has authorized taking the car across the border.
I have not been to Sveti Stefan; I have a vague recollection that you cannot actually get onto the island unless you are staying there, so that's something to verify ahead of time. Budva itself is very nice, but I thought it looked a lot like Kotor, minus the wall, and wouldn't cut Kotor short just to drive to Budva. That said, if time permitted, one could take different routes to and from Budva, returning by way of the interesting interior town and former capital of Cetinje. But verify that the road between in Cetinje and Kotor is not too tricky to be a comfortable drive; I believe someone here reported that it was nerve-wracking, but I could be misremembering.
If I were staying in Kotor for just one night and only had time to see one other place in Montenegro (recognizing that Kotor could take up all the available time), I'd choose Cetinje (if the road isn't too challenging) or perhaps Herceg-Novi. The latter is more or less on the way back to Dubrovnik.
Warning: I know nothing about parking in any of these places.
