There used to be a bus line, bus No.400 which was the Athens Sightseeing public bus. I never used it but it seems not many visitors used it either because I guess they didnt want want to spend 90' in bus seeing the center of the city without doing anything! The local joke was “what is blue, empty and does circles in the traffic of Athens?”
The line stopped operate in early June, 2010.
Your alternative is the CitySightseeing bus, a typical tourist bus with open top floor so you can take nice photos (and burnt out by the sun in summer!). You can hop on at Syntagma square. The day ticket is 18 euro and there is one every 15’. The route has 15 stops and in normal traffic you will need 90’ to return to Syntagma square again. There is an audio guide in 8 languages.
If you want to take a quick look at the highlights of Athens take the bus no400.The ticket costs 5euros, you buy it on the bus it is valid for 24hrs and can be used to other transportation modes except of those that lead you to the airport (express bus line to the airport and metro line to the airport).On the bus there is an attendant that provides you information about the places you will see.The bus runs from 09.00 till 18.00 and the starting point of this 90minutes round trip is at the Archeological Museum (Vas.Irakliou str.)
The bus stops
1. NATIONAL ARCHAEOLOGICAL MUSEUM
7. MARKET (VARVAKEIOS AGORA)
8. KLAFTHMONOS SQUARE
10. BENAKI MUSEUM
11. NATIONAL GALLERY (ETHNIKI PINAKOTHIKI)
13. NATIONAL GALLERY (ETHNIKI PINAKOTHIKI)
14. PANATHINAIKON STADIUM
17. OLYMPIC ZEUS TEMPLE (NAOS OLYMPIOY DIOS)
18. GREEK PARLIAMENT (VOULI TON ELLINON)
19. UNIVERSITY (PANEPISTIMIO)
I learned about this bus services only when I bought a map of Athens and it was mentioned in there. The cost is 5 euro and you can use it for 24 hours, on the busses, trams, metro and this unique b bus. It has a number of stops, starting at Archaelogical Museum, tehn Omonoia, Psyrri, Kerameikos, Thiseio, Monastriraki, Varvakos Market, Klafthmonos square, Syntagma, Mpenaki Museum, National Gallery, Ampelokipos, Panathenaian Stadium, Plaka, Akropoli, Olympic Zeus Temple, Greek Parliament, University and other stop at Omonoia. The commentary tells you upfront what is to be seen at the next stop and if you can get of there for any metro stations. Great and cheap way to really see this lovely city and all it has to offer.