Just got back from a tour of Croatia and Slovenia. When you're near the coast the seafood is the way to go. Once inland the food becomes comfort food similar to German cuisine--gravy, meat well done, stews, etc. Here are some restaurants that I liked. Prices are in US dollars, two people, a starter or salad and an entree, a liter of table wine before tip (which is around 10%):
Hotel Rixos/Libertas--20 minutes from Old Town. Dinner buffet. Outstanding! $100 but if you back away they may cut the price in half. Beautiful seafood salads.
Lokando Peskarija-on the waterfront behind the city hall. $60. I had whitebait and my wife had mussels in wine broth. One of the best meals if you like to eat little fried fish whole.
Proto-The best meal for both of us. $95. I had seafood risotto with squid ink and my wife had octopus stew. Great!
Atrium Hotel (Cardo Restaurant)-beautifully decorated restaurant in modern style. $65. I had veal roll and my wife had shrimp onm arugula and mozzarella. Lovely meal.
Istranka-$55. Veal in a wine sauce and my wife had an octopus salad followed by calamari. Not overly breaded but very tender.
Yacht Club-$55. My best meal of the trip--squid a la Opatija--squid on sliced potatoes with a light tomato-based sauce. Outstanding. My wife had grilled vegetables.
Kaneta-$55. A ton of roasted lamb--the lesser parts so cooked thoroughly. A lot of fat but thoroughly enjoyable. More than you can really eat. Great service.
Kvarner-actually in Lovran about 6 miles from Opatija. $70. I had the seafood brodetta (fish, mussels, squid, octopus and shrimp in light tomato based sauce)
Gostilnapri Planincu-$60. My wife had pork with a mushroom sauce and I had a pork cutlet, breaded as big as your head. More than I could eat. Comfort food to the max.
Union-$65 We both had the House meat plate. More comfort food and plenty of it. We should have ordered the lamb chops which looked great. Fabulous service.
Sokol-$70. I had mountain oysters and my wife had veal stew. Beautiful restaurant. Wild game is their main thing.
Vinodol-$50. I had the cold meats plate and my wife had pasta with truffles.
Boban-$40. We both had great pastas. In the cellar of an old building owned by a former professional basketball player. Lots of local celebrities/jocks.
Ivica i Marica (Hansel and Gretel) in back of the open air market. Beautiful interior with antiques. $60. I had beef goulash and my wife had fried bread with cottage cheese/sour cream. (I know this sounds a little strange but it's a classic for Croatia--they specialize in traditional foods.) Great restaurant.
Hope this helps. We wold recommend any of the above restaurants. I haven't bothered to mention the ones we didn't care for.