Very Beautiful - but can get busy. Most people simply visit the harbour area and walk up to the Church of St. Euphemia which is a very enjoyable thing to do with stunning views. However there is more to Rovinj. You could try the bell tower next to the church for even better views - however you will have to be OK with heights as the stairs are open, old and wooden!
Another idea id to take your swimming gear and go for a swim from the beaches. Best Location? Beaches by the stunning Hotel Lone. Park your car there - walk into town then come back after lunch and have a swim in the afternoon.