Nik's place is in a great location, no matter what you intend to do in Athens. It is about a 5 minute walk to the metro/ bus station. It is also 5 minutes from the Greek Parliament and National Garden. In the other direction, you can reach the flea market, upscale shopping, and a TON of restaurants in 5 minutes of walking.
The house has everything you need because you should be spending all of your time in the city!
A good thing to know is that Athens is pretty much asleep until 0900! Most stores and cafes are closed until then. On the other hand, everything is open so late!