We had a similar problem last March (in reverse - from Trieste to Rovinj). In the end I bit the bullet and arranged for our Rovinj hotel to send a local cab to pick us up in Trieste and bring us to Rovinj. It wasn't THAT expensive (110E) considering it extended our time available in Rovinj (it's a short, direct drive) and we didn't need a car rental for one day. You might want to ask your Rovinj hotel for a quote - it wasn't an unusual request it seemed.

Agree with staying in Rovinj over Pula - it's charming (although a day trip to Pula is also great)

Another possibility is to drop your car in Porec and catch the bus to Trieste there. If the bus schedule is not convenient, Porec would be worth a night. And it's a more direct route to Trieste (hence shorter) than going to Pula (which I didn't like) or Rijeka.

We had a nightmarish 6-hour experience trying to return a rental car in Rijeka two years ago. Yes, the office was nominally open until noon, as the sign on the door said, but when we arrived at 11 am, it was closed. And in fact looked as if it had been closed for some time, perhaps weeks, perhaps months.

No problem, we thought to ourselves, as we had six hours to sort it out before catching our 6 pm ferry south. As the ferry departure hour drew closer and closer, our efforts to return the car grew more frantic. There was no answer to our repeated telephone calls, no one returned our messages left on the number posted on the office door, and all our efforts met with dead ends.

Finally, in desperation, we threw ourselves on the mercy of the concierge at a nearby 4- or 5-star hotel. They (there were three of them behind the concierge desk) weren't surprised in the least by our predicament, implying it was a frequent occurrence. They told us with some astonishment that some travelers even tried to leave the car and keys with them at the hotel. Well, this is exactly what we were thinking of doing, which they refused to acknowledge but surely realized.

They jumped into action, began telephoning here and there, eventually managed to track down the man who was supposed to be at the rental return office, and was now in a town some 45-60 minutes away enjoying his Sunday lunch. In addition to which, the office was no longer located at the address we were haunting, but had moved some distance away.

The concierge in charge of helping us began berating the man, who finally gave in and agreed to meet us in Rijeka no later than 5 pm. When he turned up, he tried to charge us for an extra day because we hadn't returned the car by noon! We all finally settled down and he saw the sense in our complaint that we couldn't return it when agreed because he wasn't anywhere to be found.

Yes, we made the ferry at the last moment, enjoyed a delightful dinner and a comfortable overnight in our little cabin. But the moral of this story is, be aware that complications may disrupt your best plans...

Wow...this is amazing...I wasn't expecting that anyone would be able to help!

Elizabeth_S - I hadn't even thought of the taxi, but based on this site (http://www.connecto-taxi.com/en/taxi-transfers/rijeka/trieste) it looks like I can get one for 98 Euros (1h 18 min) if the bus falls through for some reason. We'll have 4 hours of buffer and I'll feel better knowing there's a plan B.

Mimar - unfortunately the Porec bus line doesn't run that day.

julia1 - We're probably going to book with Oryx via Autoeurope. Since we're going through a consolidator, we may encounter similar shenanigans. It's good to know that we should get to the nearest concierge desk sooner rather than later if there's a "be back in 15 min" sign.

I'll keep checking back in case anyone else has additional ideas or options! Thank you, thank you, thank you!!