We will be in Rovinj for 3 days. What are the best sights, towns and wineries on the Istria peninsula? We were going to rent a car but would local transportation be better?
Definitely rent a car. The best places are not reachable otherwise. We go to Istria every year for a week to ten days. My top 5 would be Kamenjak peninsula/Safari Bar, Groznjan, Brijuni, eating steak at a secret location, seafood in Novigrad. Bonus - Kozlovic winery in Momjan.
Ah, Rovinj. Beautiful Venetian architecture, beaches, Saint Euphemia Cathedral (climb the bell tower for a magnificent view.). I can't speak to a car rental since we were on a tour, but can recommend driving to Motovun- worth the 1052 steps to the top! Well known for Truffles, and Film Festival. Pula (best preserved Roman Amphitheater), Temple of Augustus...
Porec has colorful architecture, though it is very touristy (as is Rovinj). Although some of Istria's highlights can be reached by bus, you'll see a lot more, and more efficiently, by car.
Rental car allows you to drive to Grozhnjan, Oportalj and Motovun. My favourite is Oportalj and the terrace restaurant by the loggia, smaller and less crowded than the other two. Pull off at the viewing tower above the Lim canal on route and enjoy the view. Explore the back streets and walls of each hilltop town. Artist's ateliers hidden in all directions. Rovinj itself has side streets no-one visits.