Athens is ancient metropolis, and can offer classical sights, great food, a vibrant nightlife and friendly people, in addition to everything else you would expect from a city of 4 million people. The only reason to avoid Athens, is if you are only looking for beachlife, and not culture and history. For that you should probably go to one of the islands instead.
The best known sight of Athens is of course the Acropolis of Athens, a cliff that dominates the skyline of central Athens, housing several temples, including the Parthenon. Museums such as the National Archeological Museum and the Benaki Museum showcase the impressive legacy of Ancient Greece, where much of western civilzation originated. But there are several other attractions in Athens worth checking out.
Cafes, shopping and nightlife
Despite the size of Athens, there’s no problem finding charming street cafes in general, and in the area called Plaka in particular. Right next to it are the street markets of Monostiraki, usually swarming with both salesmen and tourists. Athens has both designer shops and meat market, depending on what you want to see.
During night time, Psyrri is the place to go, with numerous restaurants, bars and clubs. The nightlife is unusual in that several of the most popular clubs actually relocate to one of the islands during the summer.
When to go
The climate is generally hot in summer and mild in winter, with daily highs up to 20 degrees celsius even in January. The tourist season lasts from June to September, with crowded tourist attractions and restaurants, and baking heat, as evidenced by the fact that Athens has the European temperature record, of 48 degrees celsius. As in several other Mediterranean countries, many of the locals close shop and leave the city on vacation in mid-August. The best months to visit are generally considered April and May, when tourists are relatively few, and temperatures would still be considered pleasant summer temperatures by most North Americans and North Europeans.
How long to stay
Three days is plenty to see all the major sights in Athens itself. There’s always another museum to visit, if you have a special interest in the culture and history of Greece, but few people will feel they have missed anything, if they’ve spent those days well. That being said, it’s no problem to stay longer, and simply use Athens as a base for one or more day trips, e.g. to Delfi, Sounio, Corinth or the Aegian islands.
If you want to prolong your visit to Greece, but go someplace else as well, there’s ferry connections from Piraues to almost everywhere in the Aegean sea, the port of Athens. As an example, a boat trip to Santorini will take you 8 hours by regular ferry, or 4 by hydrofoil. A more expensive option is to take a domestic flight from the Eleftherios Venizelos airport. The domestic connections are extensive, and should be able to take you to all the better-known tourist sites in Greece.