Currency

The only legal tender in Croatia is the kuna, euros are often accepted but the rate can vary. Prices of accommodation and larger items are sometimes quoted in euro for the tourist's convenience - most are Europeans, but euros may or may not be accepted. I usually just take some kuna out of the ATM and pay with a card whenever possible to keep withdrawals at a minimum. I find it's the most hassle-free option and at least in my experience, quite cost-efficient.

It never occurred to me to try using euros in Croatia on my 2015 trip. If I had to guess, I'd say a fair number of touristy businesses might take euros (but give you quite a bad exchange rate), but government-owned entities (such as public museums and the rail system) will insist on kunas.

It is always advisable to use local currency. Sometimes they will take US dollars or euro but you will pay for that convenience through a very poor exchange rate. Remember it will cost them money to get your dollars or euro changed to their currency so they will want a little extra to cover their inconvenience.

Do not use euro's. Many small businesses refuse to accept them. The places that do accept them will markup the price significantly.
Croatia is cheap because most transactions are conducted in local currency. Try to keep it that way. :)