Opatija in Croatia

Anyone been and if so what is it like, i don't mind laid back but am thinking it might be really quite. what is there to do? I have a voucher for Crystal Holidays that I want to use so i am tied to something out of their brochure so if anyone has any suggestions on anything else. I am looking for someplace warm in september, with things to do and a nice beach or lake side for a swim and sun bathing with activities for himself while i will be relaxing.... Nice restaurants and bars etc.. the usual stuff!