We have a trip planned in the summer to Croatia. Already booked/decided is 3 nights in Porec and 4 nights in Rab, but we have to decide on one more destination for 3 nights in between those two destinations. The original plan was to stay in Opatija, but looking for any other ideas before we decide. Thanks in advance!

If you travel by car you could drive over Cres island - Krk Island to Rab, using the local ferries from Brestova to Porozina, from Merag to Valbiska and from Valbiska to Lopar.
I suggest to stay at Mali Losinj (road from Porozina).
You could also replace Opatija by Crikvenica.

If you decide to visit the Kvarner coast, consider Lovran rather than Opatija -- Lovran is much quieter and without the crowds of Opatija, and it still gives you easy access to the delightful Lungomare.

3 nights is not much for Istria; 4 nights is a lot for Rab. You might want to shift a night. Just something to consider.

Thank you! Lovran is a great alternative and not mentioned very much in the guide books.

Thank you! The island hop from Cres to Krk to Rab is a great idea!

Cres is really strikingly beautiful--I only passed through en route to Losinj, another great low-key island. but I am eager to return with a car.

thank you! I think we will spend the night in Mali Losinj, and plan to explore Cres on the way there. We do plan to have a car.