In cities, debit and credit cards are accepted but in out-of-the-way locations it's handy to have cash. Most towns have ATMs, but they may be out of order.
Restaurants Tipping is not traditionally the culture in Greece, though it is appreciated. Locals tend to leave a few coins. Depending on where you are, you can round it up or leave around 10%.
Taxis Round up the fare. There's a small fee for handling bags; this is an official charge, not a tip.
Bellhops Bellhops in hotels and stewards on ferries appreciate a small gratuity of €1 to €3.