U.S. citizens require a passport valid for at least three months beyond planned departure date to visit Croatia. A visa is not required if you hold a valid U.S. passport and travel for business or tourist trips that last less than 90 days within each 180-day period.
Croatia is a Member of the European Union (EU), but it is not yet a member of the Schengen area. As such, travelers need a passport to travel between Croatia and other EU member states.
Foreign citizens must register with the local police within 24hrs of arrival in Croatia to inform then of any change of address. If you are staying in a hotel, a place rented through an accommodation company, hostel, or campsite, the company or hotelier will automatically register you.