I have visited all of the towns you mention except Portoroz (drove past it on the highway) and my firm impression is that Rovinj is absolutely the place to go. Hiking and mountains don't come to mind for these towns, but water and natural beauty do.

Opatjia stuck me as sort of a swanky, resorty place (easily skippable for me)and Piran was pleasant, but nowhere near as nice as Rovinj. You also should see Porec, near Rovinj, as it is a pretty place and second only to Rovinj in these parts, really. In contrast, I did not warm up to Pula.

Let me say it again: Rovinj was, for me, a peak travel experience, one of the loveliest seaside villages I have seen, period.

If you stay in Rovinj, try to find accommodation in the old city -- stunning. There are few hotels there, so perhaps you need to look for private rooms.

BTW, Trieste wasn't my favorite city, either – a lot of it has that bombed-out-in-WWII-and-insensitively-rebuilt feel.

After Plitvice, it's a reasonable drive towards Opatija. Nearby, there are beautiful seaside villages, Lovran, Volosko, Icici, Medveja and also Mount Ucka for some hiking.
Distances between towns/villages in the area are short, so easy to see Rovinj, Porec, Pula, Motovun etc

I agree with Geonev - Rovinh first choice, and Piran second choice. Opatjia is OK for a visit, but the other 2 are more interesting for staying overnight.

You can check out my web page on our visit to these towns in 2006 for more detailed information. Here's the URL:

http://www.travel.stv77.com/croatia/slovenia.htm

I spent a month in Croatia recently. If you love nature, you really should visit Brijuni National Park. You can rent bicycles there -- few people do this -- and you'll feel like you have the lush island (with plenty of interesting archeological finds) to yourself. In addition, other very quaint towns are Motovun (as mentioned already) and nearby Grozjnan. I would stay overnight in Motovun but, again, few tourists do that. They generally bus in for the day. Opatija is ok if you do the coastal walk, which is lovely especially if you walk in one direction to Lovran and the other way to the super cute fishing village of Volosko. Stay in Rovinj, which is what I did, and you can do day trips to Porec and Pula.
have a great time.
Jeanine
www.jthetravelauthority.com

Yes, stay in Rovinj, but be aware that your drive from Plitvice to Rovinj will take a while...do not delay leaving in morning. Due to a construction delay we didn't get settled in Rovinj until after 4:00. You could easily take an evening to explore the town and do daytrips to the hilltowns the next day. Be sure and go to the front of the old church for wonderful views. There's a reason they have so many benches in that area!

Rovinj will be a longer drive than Opatija, however, so if you are looking for a busier, resort-filled, boardwalk type of place, then Opatija should fit the bill.

--Annie