On Easter itself, "Christos Anesti!" or "Christ is risen!" will be on everyone's lips. It may be startling if you hear it at midnight, along with the booming sounds of fireworks, but many Greek towns celebrate the resurrection that way.
Visitors may also hear "Kalo Pashcha!" or "Beautiful Easter (to you)!"
During the forty days of Lent, you may also hear "Kali Sarakosti", wishing you a good Lent.
As always, any time of year, Greek speakers always appreciate a happy "Kalimera!" or "Good morning!", or "Yasou", equivalent to "Hello". The more formal and respectful version of "Yasou" is "Yassas". It's not expected from tourists, but should be used for those older than you are or those who are in positions of authority.
May all your holidays in Greece be happy ones!