How about a ferry? Not sure what all of your options are, and there are fewer in April, but there are ferries between Venice and towns in Istria e.g. Porec or Rovinj or Pula. You could then take a bus on from there. I don't know if there are any ferries directly from Italy to Rijeka, but that would be optimal of course.
Another option is to take the train from Ljubljana to Rijeka, which is really close to Opatija (there is a train stop called "Opatija" but it's not close to the town - you could presumably take a taxi from there though or a bus from Rijeka to Opatija). Working backwards, you could get to Ljubljana from Ravenna by train or by a Slovenian shuttle service like GoOpti. It's a long train ride to Ljubljana from Ravenna with at least on connection, but you could take the train to Venice Mestre and a bus or GoOpti from there on to Ljubljana. You COULD possibly work that out as one long day, but staying over in Ljubljana (which is a lovely town) would make it easier of course. There is at least one direct train a day from Ljubljana to Rijeka, sometimes two depending on the season.