Carry some cash with you

There are two reasons for this.
The first one is practical, since there won't always be an ATM machine in every place you go. In fact, in some of the places the nearest ATM machine or bank could be miles away. Therefore, carrying some cash with you will save you time and the hassle of having to find a million different ways to pay for your food, accommodation, etc.
The second reason is that especially in remote areas, tavernas, taxis and hotels might not always have a working POS machine. Which means that you will have to pay in cash. Yes, I know what you are thinking. They should have the POS. Well, it's not always the case. So again, carrying some cash is a lifesaver.