I really enjoyed staying at Katerina's place. Aghios Nikolas is easy to get to - the flat is just a few minutes walk from the metro, and the centre of town a short metro trip away. I really like the area, it's a nice mix of busy and residential.
There are lots of families and different communities around the area (I'm learning Arabic so enjoyed hearing a bit of arabic from some of the families out for their evening stroll). It was really hot in Athens and the area's children were all playing out in the playgrounds in the evening and people sitting out in the bars and cafes. Perhaps the area doesn't have the hippest bars (although I was only here for two days so could be i just didn't see them) but they're easy to get to by metro from here and the sense of being out of the touristy area but still in an interesting place was refreshing. Lots of useful shops around too, phone shop, veg shops and shoe shops all nearby. Katerina told me where the nearest bakery and souvlaki places are and I made good use of both. The flat is very nicely set up with a well equipped kitchen and pleasant sitting room area. I stayed here on my own as I was meeting a friend a few days later, but would be a nice size for a couple to stay in. Katerina was welcoming and always got back to me when I had a question. I would definitely stay here again!