The Koenji station area has become a center for small restaurants and "Live Houses" which offer live music. It is also known for having a young population and as a center for suburban underground culture including multiple used record and clothing shops. This is actually where the punk scene started in Tokyo.  Really nice trendy area to go out both at daytime or night with a friendly atmosphere.
Dozens of yakitori restaurants, izakaya, ramen noodle restaurants, and bars can be found in the streets and alleys of both north and south Koenji, as well as under the elevated tracks near the JR station. Both the bars and the restaurants can have a very unique layout and feel to it that you definitely need to explore.
 
There is also a couple of green areas with 2 major parks including Sanshi-no-mori (蚕糸の森), Mabashi Koen (馬橋公園), and Wadabori Koen (和田堀公園). During the March- April months you can get a fantastic view of the cherry blossoms here, especially at the Wadabori Park.
The Koenji area is accessible by the JR Chuo-Sobu Line (Koenji Station) and the Tokyo Metro Marunouchi Line (Shin-Koenji Station or Higashi-Koenji Station). You can easily Access Shinjuku station which you then can access most train lines in Tokyo as well as the Shinkansen (Bullet train).