There is not a great deal of difference in customs and traditions between Croatia and the UK
It is a privilege to be invited into somebody's home for a drink, so if this happens, respond accordingly and thank your host.
Walking around in a swimsuit in public, away from the beach, is frowned upon. Although certain beaches are reserved for naturists, you are advised to keep your swimsuit on when on "classic" beaches. Dress when you go to a restaurant and wear appropriate clothing to visit churches or monuments. Women especially should not be too uncovered and a cardigan or scarf over the shoulders is a good idea. The same thing for you gentlemen, strolling around bare-chested creates a bad impression.
This is still a sensitive and painful subject for many Croatians.
If you are interested in this part of the country's past, remain discreet in public and opt for a private discussion about the war as you do not know what others may think. Don't ask questions if you can see that the other person does not wish to talk about it. Be subtle in your opinons and be careful about any comments you make, particularly regarding war heroes.