We took a morning drive from Rovinj to Poreč, about an hour away. The main attraction in Poreč, and a very important one, is the 6th Century Euphrasian Basilica, a UNESCO World Heritage site, with its breathtaking mosaics (but the rest of the basilica complex is also very interesting).
The afternoon was spent on a four-hour cruise from Rovinj up to and through the Limska Draga fjord. If you've been to Norway, well.... But it was nice to be out on the water for a few hours, and it provided some very nice views of Rovinj (which will be in a separate gallery).