EU citizens don’t need a visa for stays of any length, they don’t even need a passport providing they have an ID card. The same applies to members of the European Free Trade Association (EFTA): Iceland, Liechtenstein, Norway and Switzerland, that means you.
Many other nations don’t need visas either, but from mid-2025 visitors to Croatia (and most of Europe) will need to apply for the ETIAS travel authorisation before departing. This applies to nearby countries like the UK, Serbia, Montenegro, Kosovo, Bosnia and North Macedonia, as well as people from Canada, the US, most of Latin America, Australia, New Zealand and several Asian nations, including Japan, South Korea and Taiwan.
The ETIAS lasts for three years and costs €7. With it you can enter the territory of 30 European countries as often as you want for short-term stays (normally for 90 days in any 180-day period). You also need a passport that's valid for at least three months after the date you plan to leave. The application is approved in hours.