Maybe Koenji (together with nearby Asagaya) is the place that "welcomed" me in Japan, where I studied and lived just moved to Japan, but this little-known Tokyo neighborhood is one of my favorite areas. Koenji is a somewhat rebellious neighborhood. Alternative and a bit punk, with the many bands that perform in live houses, the vinyl shops and the many second-hand shops. Koenji is full of particular cafes, bars and izakayas and here and there there are also some shy murals. Koenji is a rebellious neighborhood, but you can also find several temples and shrines, as well as being the neighborhood where most of Murakami's 1Q84 is set, a book that I absolutely recommend reading.
Koenji Awa Odori takes place on the last weekend of August in Koenji, a colorful and overwhelming dance festival, which I recommend not to miss if you are in Japan at that time. For a snack, stop at one of Tokyo's most unique cafes, the Hattfinatt, and be sure to visit the Koenji Hikawa Shrine to pray for good weather during your trip! A short distance away you will also find the Koenji Temple, wonderful in autumn, where you will also find one of the 3 torii gates with dragons present in Tokyo.
