Koenji is well known for being a hub for hip culture. It's best associated with record stores and used clothing shops and can even be recognized by some as the setting for Haruki Murakami's three-part novel, IQ84.
Besides being a great place for the above mentioned things I read a lot about Koenji having a great small town atmosphere to it and I likened the descriptions of it to those that I had read about Shimokitazawa. The latter has always been a fun place to go for a walk in Tokyo - there was something about the place I couldn't quite pin down but I liked it.
On this trip I finally took the Chuo line to Koenji to see for myself. I didn't find any true record stores, although I did stumble upon a side store that had plenty of CDs, and used clothing isn't my thing, but the place was pleasant and was indeed as I imagined. I liked the "atmosphere," for lack of a better word, and if you're looking for an escape from the bustle of the city the streets of Koenji, at least when I went, provided a perfect escape.
I explored both sides of station, weaving in and out of side streets and shopping malls, wandering into residential districts and temples and parks. I stopped by a ramen place for lunch, stopped in every music and book store I found, and ate some ice cream at a convenience store. It was great, and I'm sure the place is even more bustling and full of life come night fall.
If you aren't like me and want a solid plan of what to see and do in Koenji, which is probably a better route anyways, check out Time Out Tokyo's 50 things to do in Koenji list and Clear and Refreshing writer Ian Martin's music-centric write-up on Koenji (as well as Shimokitazawa, which I talked a bit about earlier). Hopefully the next time I'm in town I can check out more record stores!-