We visited Athens in the middle of August. We saw weather forecast and we thought it would be really hard to survive a few days there. But when you visit famous places in the city you forget about the heat. It is quite easy to be in different part of the city due to different means of transportation. The city provides metro, buses, trams and trolley buses. However if you are located in a hotel in Plaka district, you will hardly use them. It is better to walk. There a lot to see: Acropol and other hills, National Archeological Museum and others. There is a great choice of taverns and cafes where you can enjoy just frappe or very tasty meal. They prepared Flea Market for shopping, but I would recommend three storey or Vyronos streets or surroundings of Akropolis metro station for this kind of activities.
We were advised not to visit Omonia square area due to people addicted to drugs. However, we were there once during the day and it was rather safe. Close to Omonia square you will find three storey shop called Metropolis where you can by CDs or DVDs with Greece music or movies.