Plitvice Lakes National Park is well connected to Zagreb by bus, and I think Split works, too (though I've always traveled from Zagreb). Rovinj would be much more difficult, because you'd be cobbling together an itinerary involving at least two different bus companies with a change in Karlovac. It would be quite a slog. The BusCroatia website has some helpful information. If you decide to attempt this, get confirmation of the schedules once you get to Rovinj, and build in a buffer so you aren't dependent upon the last bus of the day from Karlovac to Plitvice.
I do not recommend one solution BusCroatia offers--a one-day bus tour from Rovinj--since it would not get you time in the park outside of day-trippers' hours.
I suggest checking out the cost of a two-day car rental from Rovinj to Zadar/Sibenik/Split (if that's where you are headed next after Plitvice). There is a fringe benefit in that with an early start you'd have time to see a bit of inland Istria. Groznjan and Motovun are both picturesque.
You might not absolutely need the second night at Plitvice.
I have no recent experience with Croatian ferries.
