This article is from the Greece FAQ, by Nikolaos (Nick) C. Fotis, firstname.lastname@example.org with numerous contributions by others.
From: email@example.com ("Christina C. Christara")
1) Don't be disappointed if you find customs/passport officials
"heavy" and impolite.
2) Do not ever show your 5 fingers open from the inside part
of the hand. It is considered a bad gesture. But the outside
part of the hand is ok.
3) Don't pay more than 1500-2000 drs for taxi from Athens airport
to city (unless I am very out-of-date). Sometimes taxi drivers
ask people who look Americans or similar more money than they should.
4) Greece is relatively safe with respect to crime, theft, etc.
(This does not mean that you sleep on a bench and leave your wallet
in open view...)
5) Public transportation is relatively cheap; in some cases crowding
6) You can buy beer, wine, etc, anywhere anytime, you don't need
any id/age proof.
7) 2:30-5:00 pm is considered "quiet time", mainly in the cities,
but in hotels too. Don't make too much noise during that time.
(People have a big lunch around 2-3pm, and then take a nap.)
Of course, after 12 midnight till 7am is also quiet time.
8) If you drive, take care of pedestrians (and other cars too).
They tend to cross streets "illegally".
9) Few roads in Greece are straight. So if you plan to drive,
don't take into account barely the kilometres, but also whether
you are crossing a mountain, etc. (like in Delphi).
10) Eat a lot of fruit. They are in general of better quality
than fruit found in North America (more fresh, more tasty, more juicy)