Elli and Katerina's apartment is small but very cozy and comfortable. It's clean and tidy and has all the utilities that someone may need (e.g. cooking utensils, towels etc.); it has a very nice balcony with a little table where you can have lunch and enjoy the warm Greek sun!
It's located in the neighborhood of Petralona at just 2 minute by foot from a metro station so you can easily reach every interesting spot in a while (metro service is very efficient).
I really suggest this place and I will surely come back here when I will be again in Athens.