As the meal will most likely go on and on due to conversation, remember that accepting more food is a great compliment and when you do finally finish, eat everything off your plate and place your napkin on the table next to your plate to indicate that you are finished. This will probably be followed up with a cup of coffee, which may last another half hour.
If dining out, you may be asked to share a table with others if you're alone or in a small group. To summon the server just make eye contact and if you invited everyone out for dinner, be sure to pay for everyone as the inviter generally pays for everyone. If, on the off chance, you are asked to dance, which is not as uncommon as one may think in restaurants, getting up and dancing will be good form to help build relations.
Tipping in Greece is odd as there is a mandatory service charge on the bill (although it may be included in the menu price, it typically is not), but tipping in addition to this is still expected. The additional tip should be about 10% for good service.
Drinks
The most popular non-alcoholic drinks in Greece are various coffees. Strong coffee is common as is frappe, a foamed iced coffee. Other non-alcoholic drinks are also readily available throughout Greece, including juices, soft drinks, tea, and milk.
The most unique alcohol and the one that seems to be forever associated with Greece is ouzo, which is an anise seed liquor. Tentura (a cinnamon flavored liquor) and metaxa (brandy and wine) are also local drinks worth a try. Greece also has a large number of wines, which has been an integral aspect of Greek culture for millennia, although are few are popular abroad. Popular international drinks are also widely available in the country, including wines, beers, and hard liquors.
There is no consensus on the cleanliness of the tap water in Greece. Generally speaking, the tap water in Athens and on the mainland is safe to drink, but in some areas (particularly some islands) the water quality is poorer, perhaps unsafe, so should be avoided. Additionally, there seem to be outbreaks in the water source from time to time so the best course of action for a short stay is to be extra cautious and avoid the tap water entirely. If you do decide to drink the tap water, remember that many people may have troubles adjusting to the local water, as it will most certainly be different from what your system is used to.