Opatija was the last stop on my visit to this part of Croatia and I really liked the town. Upon arrival in the town centre, I recommend first taking a long walk along the famous promenade called Lungomare. It's a nice way to see the centre of the town while enjoying the waterfront. If you have time, you can discover that 12 kilometre long walk or you can focus on a specific part of the town.
I stopped to look at the beautiful Maiden with the Seagull statue, before heading towards the town centre and passing the Angiolina villa where there are great pictures to be taken of the garden. In Opatija, many of the splendid houses in the centre were converted into hotels. I especially liked the pastel-coloured buildings and the Art Nouveau style which gave a sense of character to them all.