I stayed in Opatija in the early 1970's. when the country was Yugoslavia .Opatija was a lovely, grand old resort which had been used by European royalty and our hotel, the Kvarner, had been built in the 19th century. The town has probably changed a lot in the last 30 odd years.
We combined our holiday there with a week at the Plitvice lakes - which were (and still are I believe) wonderful.
We went on many day trips from Opatija and also from Plitvice. A wonderful holiday.
Been more recently - 1997 - so since the war. It (was) still a wonderful resort not frequented much by Brits. It was built as a coastal resort as part of the Austro-Hungarian empire in the 18th century. Well worth it.