Nikos' apartment is ideally located near all the important attractions and near a metro station. So basically, you could visit all the main attractions by foot, with the exception of the (amazing) Archeological Museum or the Lycabettos Hill. Athens itself lends to exploring by foot thanks to large pedestrian area and small streets (at Plaka etc.).
Nikos' apartment is not large but more than adequate for 2 persons. It is quite modern with reasonably equipped kitchen, shower and WC.
Best of all is Nikos himself, who is very friendly and picked us up at the metro station. Nikos has also prepared a comprehensive info sheets about everything you need to know about Athens and the neighbourhood (restaurants etc.).
To sum up: very nice city, nice apartment and great host.