I studied abroad in Athens for four months, and a few tips my professors gave us were:
1. Don't smile or stare at anyone, it's considered flirting to do that.
2. Bring a map with you, but don't stop and look at it in the middle of a sidewalk, move off to the side.
3. Don't hold your hand palm up to anyone, it is the Greek equivalent of giving someone the finger basically.
4. Be careful crossing the streets, especially if you're not crossing in a crosswalk.
5. And don't pay any attention to the stray dogs unless you want to form a mini parade with you at the front.
On the streets the Greeks mostly keep to themselves, but if you walk into their place of business you will find that they are very kind and generous to everyone. Bakeries, and pharmacies, are on almost every block, although there are supermarkets where you can buy food as well, but the bakeries in Athens are worth going to at least a dozen times! If you don't have time to enter a shop to look for something quick to eat, kiosks are the way to go, although priced a little higher.
My favorite way of transportation is the metro, it is cheap and easy to use. You can purchase tickets either at any metro station or at a kiosk. Once you stamp your ticket it is valid for an hour; so if you get off at one stop and return to the metro within the hour you don't need to purchase another ticket. Metros and buses use the same ticket, where as the tram uses a slightly more expensive ticket. Buses are decent to use, but you must watch your money at all times, many of my fellow students had their money stolen when they were taking the bus. This happens on the metro as well, but the buses are usually more crowded. If you want to get to one of the beaches the tram is your best bet, since it goes right along the shore. Many people ride mopeds, and the parking throughout the city is ridiculous since many people double park.
My two favorite days of the week in Athens were Fridays and Sundays. On Fridays they have farmers/fresh markets in almost every square where you can purchase fresh fruits and vegetables, fish, flowers and even clothes. They usually run from early morning until 2pm.; my one professor said to always go near the end when they lower the prices to try and sell everything. Sundays you can go to Monostiraki, a part of Athens, and go to the flee market. Besides souvlaki and a few fruit stands, the flee market doesn't offer food. Here you can find almost everything else; shoes, shirts, beads, chandeliers and old book stores, to name a few. Even though Athens has McDonald's and Domino's, I strongly suggest finding a local taverna and trying their food and wine. When it gets warmer out, around early May, you can go to the open air cinema in Plaka, another part of Athens.
The open air cinema is just like a drive-in, except instead of sitting in your car you sit on the roof of the theater in rows of seats. They even have a concession stand in the back. The theater also sells movie posters, in both English and Greek. One thing that tourists may not know is that all ancient sites, the Temple of Olympian Zeus and the Acropolis for example, are free on Sundays.
Haha! Its funny to hear all this about Greeks (being Greek myself!). Very nice guide!