Currency
Despite joining the European Union in 2013, Croatia will not join the eurozone until January 1, 2023. Croatia will have to go through a lengthy vetting process if they wish to join the European Monetary System. However, EU countries are not required to participate; the United Kingdom is possibly the clearest example of an opt-out country. So, when you arrive, leave your Euros at home and pick up some kunas, Croatia's national currency.
The European Union is a supranational body that establishes policy on a wide range of problems, including trade and human rights. There are numerous advantages for countries who opt to join the organization, but not everything falls into place at the same time or even at all. Meanwhile, continue to use the Croatian kuna, which you may acquire in advance outside of the country or at banks and ATMs after you arrive. Although some businesses and restaurants accept euros, you will receive change in kuna, and not at a very advantageous rate.