I was only in Opatija and Rijeka, and I like them very much. Opatia is a small town or maybe even a village that was holiday place for a lot of royal families back in the days, and Rieka is very nice, too.
I love Porec. It's a pretty little place, with a harbour and market, and a really lovely big park (you can get a little tourist train through it). The "beach" runs alongside it. It's a man-made bathing platform, with lots of kids' swings, slides, etc.
I've been to Rovinj and liked it, also Dubrovnik which gets absolutely packed with tourists from cruise ships there. The buses are always packed, and nobody queues! They all push in front to get on. There are loads of shops, cobbled side streets, outdoor concerts, and boat trips.
if you've got under 10s don't miss the dinopark at Funtana. Cheap as chips and a health and safety black hole, but lots of fun. Poorly made, well-worn model dinosaurs in a disused quarry with ricketty wooden walkways, plus mini fairground style rides which you operate yourself via the on/off switches. All pretty dodgy, but lots of fun with small ones (pretty sure older kids would not be impressed though!) dinopark.hr/en/opening-hours-and-prices/
Rovinji is a lovely town- we went on a boat trip there. Dinopark was amusing! One tip I have is not to wear anything really brightly coloured. I did and was stung by an enormous weird flying insect (sort of black bee type thing but not a bee) which caused a huge reaction on my leg!
Rovinj is a gorgeous base, lovely old town, great restaurants and swimming spots. Pula definitely worth a day trip and we also loved Brijuni island - bring a picnic and snorkelling gear and head to the bay with Roman ruins .